With my choice of getting the Alienist prestige class, once I reach the 5th level of that class, my familiar will gain the Pseudonatural template, which will make it extremely strong in many aspects… except its Hit Points, since familiars are supposed to always have the master's Hit Points divided by 2.
How can I increase that? Of course, my familiar will have a lot of resistances, but at that level, it surely won't live long against an enemy that can bypass them if it only has half of the HP of a caster with d4 Hit Dice. Plus, as an outsider, at that point it'll lose the advantage that other familiars have, of being resurrectable with only Raise Dead at no penalty.
Best Answer
You are correct regarding hit points of a familiar:
So what can you do?
Increase your own hit points.
This can be achieved by feats, such as Toughness and Improved Toughness.
This can be achieved by magic items such as Manual of Bodily Health.
Keep your familiar away from danger.
This can be achieved by magic items such as a Familiar's Pocket.
This can be achieved by having a flying familiar such as a raven.
Do not think of your familiar as an animal companion.
Use your familiar as an extension of yourself, rather than a combatant in its own right.
Equip your familiar with items that will boost its AC.
Browse Dictum Mortuum's Familiar Handbook.
The guide is extremely in-depth.
The guide is very informative.
The guide gives pros and cons of almost all "official published" familiar.
Take advantage of share spells.
Chances are, you are going to cast Mirror Image and Blur on yourself; it also affects your familiar.
Chances are, you are going to cast Fly and Mage Armor on yourself; it also affects your familiar.
Key Note: The things you do to increase your own resilience, mitigation, and immunity, will most likely apply to your familiar as well. You won't last long duking it out with a dragon; neither will your familiar. That is what the Barbarian is for. Let the Barbarian take the hit, while you and your familiar do fine on the side line.
From Comments
That is +35 natural armor, not insight, which does nothing for touch AC. Despite the stat increases, your familiar will still not be a replacement for other summoned creatures that can tank for you. Your familiar, will not have the hit points to mitigate damage. There are no temporary hit point choices that are will really prevent anything when a 10th level barbarian can do 150-300 on a charge attack.
Invisibility (Greater), is a wonderful thing, so is Blink. Not getting hit and killing your enemies before they hit you - is the secret to 3.5 Edition.
Also, since there are going to be tentacles involved - grappling is going to be possibly a thing. An octopus familiar, becoming an outsider (extraplanar) would give that many more tentacles, and not require to be underwater to breathe. Combined with its higher strength, and Evard's Black Tentacles, it could hold an opponent off without too much harm to itself. Yes, Freedom of Movement would cancel all of that - but there is nothing that states that its Ink Cloud only works underwater (silly, I know). There is information about being a Grapplemancer here. Vampiric Touch (maximized) will give really decent temporary hit points if that is really a concern of yours. The Fearsome Grapple spell will also be great to use.
Don't forget about summoning! Summon all of those 'aliens' to keep things off of you and your familiar. Trust me...