#Naming items not in your inventory

The most important thing to note is that identifying items in one game only helps you in that game. If ruby rings were rings of invisibility last game, but you died and started a new one, you know NOTHING about them. The exceptions are the trivial ones: potions of water and blank scrolls/spellbooks.

The second most important thing about identifying items is that each item can be blessed, uncursed, or cursed. If your role is Priest, then the blessed/uncursed/cursed status is automatically revealed to you for all items. If you are of any other role, then you need to utilize alternative methods to determine the status. The available options are:

• You can figure out which items are which by dropping them on an altar (as long as you are not blind, so that you can see the flashes of light).
• Reading a scroll of identify will also point out the status of the item.
• As mentioned in Durathor's answer, pets will avoid walking on cursed items that are on the ground, only moving "reluctantly" if they ever stand on it. You can use this from the start to avoid an early demise from using cursed equipment.

Blessed items and uncursed items are much safer to use than cursed ones. Don't drink any cursed potions, read cursed scrolls, or put on any cursed rings/armor unless you know exactly what you are doing.

Rings and armour

Conversely, if you have rings or armor that you know is not cursed, you can put it on and take it off right afterwards. For certain types of equipment (ring of invisibility, gauntlets of dexterity, boots of levitation, etc) this may identify it immediately. Others will have clear effects, allowing you to #name them yourself. (Ring of conflict, gloves of fumbling, etc) This will also tell you how heavily enchanted a piece of armor is.
Note that certain types of rare headgear can still cause problems, even if they are not cursed. The helm of opposite alignment will lose you all your divine protection, and will block you from doing the quest. Anything that looks like a "conical hat" should be avoided unless you are a wizard, in which case you may want to wear the expensive type.

Potions

Potions provide you with some interesting options.

• Clear potions are always water. However, they may be blessed (holy water) or cursed (unholy water), which have important uses. You can find out what kind of water you have by dropping the potion on an altar (they won't break, promise).
• You can try #dipping a unicorn horn into a potion. Most bad potions will be neutralized this way. If you dip a unicorn horn into a brown potion, and the brown potion clears/changes color, don't drink any brown potions.
  However, just because the potion did not change, does not mean it is safe to drink. Unicorn horns don't detect everything. (sleeping and acid, for example). Also, a potion of polymorph will cause you to lose your unicorn horn as it turns into something else. This can be prevented by dipping any other item into the potion first.
• If you find a food shop, you can try dropping your potions there. Anything that the shopkeeper offers to buy from you is a "food potion." In practice, that means water, fruit juice, or booze.
• Monsters will drink helpful potions (speed, healing, invisibility) and throw harmful potions (paralysis, blindness, acid) at you.
  When in doubt, wait for this to happen instead of drinking random potions. The penalties are much less severe, especially if the potion was cursed.
• If a potion was dropped by a nymph, and it's not one she stole from you, chances are that it's object detection.
• The potion of oil is unique in that you can apply it to light it on fire like a lamp. No other potion is a valid target to apply.

Scrolls

For scrolls, I typically end up relying on price. As with all items, more expensive scrolls do 'bigger' things. Some specifics on scrolls:

• First level of Sokoban always has two scrolls of earth in the same spot.
• If you find a scroll in a one-square "closet", it will be a scroll of teleportation.
• Scrolls of identify are really cheap. Cheaper than blank scrolls. This makes them pretty easy to identify.

For more information, including a very detailed price ID guide, see http://www.steelypips.org/nethack/id_faq.html

There are four main things that help in hording items: getting a bag of holding, keeping a stash, travel improvements, and value assessment.

Bag of Holding

These can be purchased in tool shops and other stores, can sometimes be found lying around in the Gnomish Mines, and there's a 50% chance you'll get one at the end of the Sokoban levels. Refer to this map for reaching the Mines and Sokoban, but simply put they're fairly early. The earlier you get one, the easier everything becomes.

The bag of holding is a container, which automatically expands your carrying capacity. Unlike your inventory which is limited by the number of letters in the alphabet, an infinite number of items can be stored in a container. Of course, an infinite item quantity is an infinite weight, which is where the second benefit comes in that a bag of holding reduces the weight of its contents. An uncursed bag of holding reduces the weight of items by 1/2, while a blessed one reduces the weight by a whopping 3/4!

Bags of holding make it a lot easier to both carry all the essentials you need to survive in the dungeon, as well as traffick heavy goods across long distances. Hope to get one as early as possible. However, there are some dangers involved with what you put into a bag of holding: read up on them here.

Keeping a Stash

It's indeed unreasonable to try and carry everything you can. So what is important is to have a place to safely store your stuff. Keeping a stash often is tasked with a few points.

1. Scare monsters away - monsters that might pick up your stash items can be scared away by either dropping a scroll of scare monster on the stash, or by engraving "Elbereth" onto the ground. Ideally Elbereth written permanently via a wand of fire or wand of lightning. (Since 3.6.0 though, Elbereth only works on the square you are standing on, not on stashes.) So use one of these, or hostiles can do things like pick up and use your weapons, wands, potions, etc, or more dangerously gelatinous cubes can eat all your stuff.
2. Use a container - Not every monster respects Elbereth, and the majority of creatures that don't happen to be the kind that pick stuff up. Storing it in a container prevents these creatures from accessing your stuff. Pick something like the many chests you see lying around. Just be warned that most containers are of an organic material which gelatinous cubes will eat, destroying everything inside - so don't forget Elbereth!
3. Keep in a safe place - Elbereth and containers only protects against creatures themselves, not stray bolts of energy. You want the stash to be in a place that will ideally see no monster activity, or not be in danger of damage. Many people pick the first level of Sokoban, as its monster spawn rate is very low (and the Eye of Aethiopica artifact provides a very quick warp to Sokoban when Invoked). An alternative method is to store your stash in a one-tile room behind a locked door. Also consider using boulders, which block most monsters from accessing it.
4. Keep easy access - Don't create a stash in a remote place if you can avoid it. Instead, try to stash it as close to stairs while still being out of the way of combat. Combine this with mining short-cuts through levels if you have a pick axe or mattock, and it'll be a lot quicker to return to the stash once you have better identification methods.

These are the main points of stashing. All it needs to do is hold onto items and keep them safe.

Travel Improvements

Keeping a stash is worthless if you can't use it! You need to have better methods of travel across the levels. This isn't really limited to hording items, but it's something I can cover as a relevant tangent. Just consider 3 commonly accessible methods of easing your travels (I'm not going to cover the Eye of Aethiopica since it's rather specialized and already covered up above).

• Level Teleportation - One of the best methods to reduce travel time is level teleportation. The easiest method is by reading a cursed scroll of teleportation. Combined with teleport control in some fashion, this lets you very quickly reach the level that your stash is from, and then return to where you need to be. Very risky without teleport control, however.
• Teleportation - Standard ease of travel when moving across a single floor. It can also speed up actually reaching your stash, and then returning to the stairs. Not as risky without teleport control, but can be more annoying and time-consuming.
• Digging - Pick up a pick axe or dwarvish mattock, and just start taking out walls. Convert as many floors to a straight line path from stair-to-stair. The less corridors you have to navigate, the less resources spent. While digging holes to fall is an option, do note that where you actually drop is random so building a complex "down elevator" doesn't quite work as well as it may seem in theory.

Value Assessment

The final point to knowing how to effectively horde items is knowing what to horde. Equipment, such as weapons and armour, are often useless to keep. They sell pitifully compared to their weight, and you rarely need to keep any equipment that isn't immediately useful to you. Also try to figure out the usefulness of items by unconventional means. Reduce not just how much you have to carry, but how much you need to stash as well.

And finally, consider carrying no more than one copy of heavy items like potions when unidentified. Identifying one serves to identify them all, and simply rely on one or more stashes to keep the extras.