I have my own personal little "Checklist" that I mentally go through when I'm going to be doing an instance. I thought I would share it with you guys in case anybody might find it useful:
Before Heading to the Instance:
-Ammo: Make sure your quiver/ammo pouch is full of the best arrows/bullets you can buy. If the instance is especially long you may want to bring a couple extra stacks in your regular inventory as well, but to be honest I haven't found that I really need to do this in regular five-mans.
-Pet Food: I never bring anything less than a full 20-unit stack of pet food to an instance. If I know it's a particularly hard instance, or one that is long, or that I haven't done before, I'll bring 25 pieces of pet food.
-Hunter Food: Unless you've got a mage handy, bring some food and drink for yourself.
-Repairs: Even if your durability is still at 90%. Get it to 100%.
-Bag space: It sucks to spend most of the instance trying to juggle what loot you want to keep and what loot you want to toss because you only showed up with three empty slots or something. (Trust me on this one.) Vendor stuff, put it on the Auction House or in your bank, or mail it to an alt and give yourself ample room for the things you'll pick up!
-Potions/Elixirs/Buff Food: If you have some, bring it! Bring some for your pet too!
-The Right Pet: You probably don't want to show up at Heroic Mech with the level 19 ghost saber that you just tamed a couple minutes ago. (Just a hunch.)
-Spec'ing the Pet: This is sort of an optional one and some people don't like to re-spec their pet for every instance. But it's cheap and it's not a bad idea to give your pet extra nature resistance for Steamvaults, for example.
At the Instance:
- Growl is OFF. If you have Cower, it's ON. If you have Screech, I myself like to turn it OFF (just because it causes threat on multiple mobs; I'm not sure if it breaks CC or not. Oh and a lot of tanks really hate the noise).
- Tank assist macro (feels an urge to write a macro article brewing) is set to the current tank.
- Pet is set to PASSIVE.
- Buff up: Is Aspect of the Hawk up? (I know I've forgotten once or twice). Have you used your food and agility elixir? Did you feed your pet some Sporeling Snacks or Kibler's Bits?
- If it's a group of people I haven't worked with before I usually inform them that I am able to trap and that if they want me to trap something they should stick a blue square on it, and I'll take care of it. Sometimes they'll want to use a different symbol other than my usual blue square, that's fine with me so long as they let me know (they always do).
Well, that's my list. It seems long and scary but it really doesn't take too long to do, especially if you're like me and you like to be super-prepared and you go replenish your supply of ammo and pet food and repair every hour or so.
Everybody's experience is different when it comes to this type of thing so develop your own list and find out what works best for you, but for the most part I think you'll have a similar list. Do take note though that I have never done a real "raid" before so the list might be a bit different if you are further-progressed than I am (and I imagine a lot of you are).
Remember, "failing to prepare is preparing to fail". Thus sayeth some random inspirational poster. =P