Focus Fire: Don't spread out damage any more than you have to. It's far more effective for most of the monsters to concentrate on a single target. If you can get a player down, that's that much less damage per round Team Monster is taking, and the party is suddenly on the defensive to try to save the downed player (only particularly nasty monsters should take the opportunity to coup de grace an unconscious player). Which player should you target first? That leads to the next question...
Target the Squishies: Monsters should always focus fire the available target with the least current HP & lowest defenses. That's who they're most likely to successfully take down. Brutes in particular benefit from attacking low-defense targets, since they have high damage but low attack. Note that I said available target, though. If a sticky defender is guarding a doorway, don't try to rush past him to get at the wizard. But if several monsters can get at the wizard...
Weigh Cost/Benefit for Marks: If a monster is marked, you shouldn't have it automatically attack the fighter or always ignore the mark; only ignore the mark when it's worth it to do so. How much damage as a percentage of the max HP are you likely to do to the defender, versus how much are you likely to do to whoever else you could be hitting? If you're probably going to be more effective against someone else, how much HP do you have left and how unpleasant is the defender's mark punishment? Remember that any target that has defenses that are lower than the defender's by more than 2 is easier to hit than the defender even when the monster is marked.
Flank Them: Have a couple lurkers or skirmishers come in behind the party (they were hidden, used secret doors, or whatever). It's very hard for a single defender to stop attackers coming from 2 different directions. Getting a couple melee units in among the party's ranged backline makes it much harder for the party to focus fire, which means monsters last that much longer.
Control, Control, Control: Make good use of controllers and leaders. Unlike damage, control should be targeted at the characters it will be most debilitating on. Immobilize/slow are for melee attackers, blindness & daze should be dropped on the biggest threats, daze shuts down most defender mark punishment (except paladins), and so on. Control has a huge effect on the difficulty of a fight.
Protect Your Squishies: Artillery and ranged controllers, and when possible leaders, should be stationed towards the back. Use soldiers and minion packs to tie down melee players so that they can't get to your ranged units and to keep defenders busy so lurkers, skirmishers, and brutes can slip past to the party's weaker ranged units.
Use the Terrain: Have your ranged attackers hide behind pillars (remember, if you're directly behind a one-square pillar, you have cover but enemy targets don't). Remember that monsters can use each other as cover if necessary. Position your frontline and backline to maximize the amount of difficult or dangerous terrain melee players have to go through to get past the frontline to the backline.
Alpha Strike: Winning initiative helps; if your group simplifies initiative by having all the players go in order then all of Team Monster go, or vice versa depending on which team won initiative, then winning initiative really helps. Monsters should use any encounter or recharge powers they have early & often. It's better to use it at the start of the fight in a slightly less-than-ideal targeting scenario than to save it for later and end up dying without using it. Even if you can't alpha strike (and it's admittedly much harder for Team Monster than it is for the party), it's hugely beneficial to prevent the players from alpha striking. To give an example, with a proper alpha strike the group I play with have often killed or incapacitated (prone + 1 square away for melee enemies, prone & around a corner for ranged enemies, blinded, etc.) over half the monsters in an encounter before any monsters got to act; the record is 7 monsters (no minions) getting a total of 3 attacks before the fight was over (not 3 per monster; Team Monster as a whole got in 3 attacks before the fight was over, including misses).
Retreat & Reinforce: If a group of monsters that are part of a larger faction in the area are getting their butts kicked, have a few of the remaining ones retreat to try to join an allied group a room or two over. This not only makes the next fight harder without technically altering XP budgets, it has the added bonus of reducing the boring mop-up phase.
Encounter Building: Picking the right monsters to use can make a huge difference in an encounter's difficulty, even when your options all have the same total XP budget. Here are some things to remember:
Frontline & Backline: Try to have a mix of frontline monsters that are designed to keep melee players busy and backline monsters that are designed to hit hard. You will especially need a good frontline if most of your "backline" monsters have to stay in melee range (brutes, some skirmishers and lurkers).
Monster Synergy: Pick monsters whose abilities complement each other. If you have a skirmisher/lurker that does bonus damage to slowed foes, find a controller with a nice AoE or save-ends slowing attack. Daze & prone is another nice combo, especially against melee players; immobilize & forced movement is also strong against melee players. If you have several monsters that give a bonus to adjacent allies, go heavy on melee monsters so more of them can benefit.
Terrain Synergy: Are there a lot of hazards players will have to avoid? Bring lots of forced movement to try to shove them into it. Lots of terrain sources of elemental damage? Pick monsters that resist or are immune to that damage, so they can move through it with impunity. Lots of difficult terrain or features that block movement but not line of sight? Go heavy on ranged monsters so you can pincushion the players as they move to engage.
Control Is Good: Control effects, used properly, will generally be more effective than damage if you have a couple high-damage monsters to take advantage of them and you have some blockers to keep your control monsters from dying the instant the players notice them. Generally speaking, a good ratio is 30% blockers, 20% damage, 50% control. This doesn't mean use nothing but controllers, though: remember that lots of non-controller monsters offer decent control effects, especially as you start getting into paragon tier (by epic, almost every non-minion has some sort of control).
Don't Use Pre-MM3 Solos: This one is admittedly a little specific, but it's good advice nonetheless. Before MM3 solos depended entirely on having lots of HP to survive, but this meant a typical solo fight entailed the party stun-locking the solo for a round or two while the strikers went nova on him, typically ending the fight with little or no damage inflicted on the party. I've actually seen a mid-heroic solo killed in the first round by a single player (admittedly he critted with a daily power, action pointed, and then critted with a second daily power, but still). MM3 and later solos tend to have more ways to negate or ignore status effects (especially stun/daze) and more ways to interrupt multi-attack chains, thus giving them considerably more survivability.
Use Higher Level Monsters When Possible: All else being equal, 4 level+1 foes will generally be a harder fight than 5 level-1 foes. Monster and terrain synergy definitely trump this one, though.
That said, not every monster is a tactical genius. Animals in particular will tend to just concentrate on whoever is closest or doing the most damage to them, and will almost always ignore defender marks (though most predators will attempt to start the fight by going for whoever looks the weakest). Dumber and more cowardly enemies (goblins, kobolds) will usually attack in a big pack, and will almost always respect defender marks.
I play online almost exclusively these days, using Mote. I DM two campaigns, and this particular issue comes up often enough.
First, you have to keep things interesting. Try to design combats with more than just "attack roll -> damage roll". It's not a simple task, but it's really important to make combats interesting. Add some ranged enemies, healing enemies, mages, etc. Throw in environmental hazards, or effects which occur between turns. Give the PCs a time crunch to win. Give them a reason to be concerned.
In terms of process, I have found that the method of forfeiting turns does help. If they take too long, they lose their turn. However, I found that rewarding them for taking swift actions is far more effective. I give my players a +1 circumstance bonus per five levels to any rolls they make during their turn if they begin taking actions within fifteen seconds. I call this bonus "seizing opportunities." It works phenomenally. That, plus some recommendation for the players to plan actions during someone else's turn, means that they are likely to have their plans ready and in motion within those fifteen seconds. The bonus is big enough to be worth striving for, without being so big that it's not worth sacrificing for the occasional tactical overview. Players are much more likely to confer with each other this way. This has sped up our gameplay immensely.
You can adjust the bonus, time limit, and everything else to whatever works for you. I just insist that rewarding players is a more effective motivator than punishing them. It also makes them feel good and have more fun. That is the most important thing.
EDIT: Now that I think of it, this is also a pretty effective way to speed up offline tabletop combat, for anyone else who might be doing one or the other or both.
EDIT EDIT: One other thing I should mention is that when playing through an online tabletop you usually have computing tools available. Take advantage of them! The human brain is great at lots of amazing stuff, like creativity, imagination, reason, and emotion. One thing it has a hard time with is mathematics. Fortunately, computers rock that stuff. It helps to shave more time off of a turn if you have as much action content (rolls, initiative order, etc.) hard-coded beforehand. Dice macros are a simple example.
On a non-computing basis, it also helps to have actions listed out in front of the player. I know there was a lot of hate around 4e's action system, but having a sheet of actions clearly delineated in front of the player was a huge advantage for newbies. Having some clear options in front of them helps remind them of what they can do, and thereby lets them make a decision sooner.
Naturally, you do not have to be as hard-and-fast as 4e. You can even go as simply as reminding the player of what they can do in your game, or give them some soft suggestions.
Best Answer
This echoes some of the other answers, but I wanted to weigh in:
Flesh them out
Create an actual NPC village exactly the same way you would create one of a non-monstrous race. Create villages, shops, story arcs, government, etc.
At the humanoid village
Try to keep most of the "crimes" of the monstrous race to be non-specific. Have the villagers say things like "there's one that I caught stalking my daughter!" (reality: they both happened to go to the river for water at the same time). Maybe give one actual issue that people keep citing but upon deeper reflection most of the other "stories" are really misinterpretations or outright racism.
Do not create random encounters
The party's first encounter with this group (I'll say lizardfolk from now on but obviously whichever race you choose), won't be "okay now the PCs are going to meet 5 random lizardfolk, rolled from XYZ table with ABC treasure", but it will be "the village has patrolling guards." They will act like guards do. They won't attack unless attacked first, they may be interested in why the party is there and ask them questions, but they won't attempt to detain the party unless the party gives them cause.
If you really want to create an effect where "they seem hostile at first, but the party begins to slowly realize..." you can create a "crime" for the party to commit by accident. For example, if the party trespasses in someone's farm or, worse, an egg creche, without realizing it, then the guards will try to arrest (but not kill) the party, and the party, expecting combat, attempts to fight back.
The guards, when some of them are killed, the rest would probably flee rather than fight to the death. Flesh out their inventory with things that guards might normally have, like whistles, standardized weapons and uniforms, etc.
Have the local government respond the way a lawful good government would
If this is the beginning of the campaign, the party will almost certainly not be able to overwhelm an entire large, organized settlement. The reason that civilizations go the organized route is that it keeps the people alive and conserves resources (as opposed to the chaotic evil that constantly wastes resources fighting each other). If it's one town, there will still probably be some people that are higher level. They may eventually capture the party and give them a trial and counsel and the party may realize that it's not a sham trial, and that "oh crap, we just killed some people with families etc. what have we done?"