Working with a Warrior

• Warriors actually do best when being beaten on - their Rage, increases when damage is dealt and when taken. Check your skill's tooltips and avoid using anything that increases 'threat' where at all possible, as this will make the enemies more likely to turn to you.
• Don't run from an enemy that is attacking you. This is very important: call for help, but stand still! Usually the first thing people do is to start running to get away, but this will make it more difficult for a warrior to help getting that mob off you. Warriors generate threat quite quickly and can Taunt to lure the enemy into attacking them, but these actions take a few seconds of uninterrupted fighting to charge up and have an effect, and this is difficult at best if the Warrior has to chase the enemy down first.
• If you do feel the urge to run from a mob, at least run to the tank; that way the mob is inside their melee range, so they can pull it off of you.
• Buffs to Stamina are particularly useful to the Warrior, greatly increasing available HP (Power Word: Fortitude, etc.). Buffs that increase the damage done, either via Strength, by the weapon itself, or damage shields (Thorns, etc), will help the Warrior maintain aggro.
• Avoid using damage absorption buffs (i.e. Power Word: Shield, Blessing of Protection) on Warriors, as this can slow the rate their Rage charges up. Best saved for when the Warrior is unable to be healed normally for any reason.
• Avoid using pets that actively Taunt themselves; Bear Growls, Voidwalker Taunts, etc. In particular, 'Rogue' or 'Mage' type pets work best if someone else is on tanking duty. If possible, turn these abilities off. Shamans should avoid using Stoneclaw Totem.
• Warrior Crowd Control is practically non-existent, being mostly Snare-type movement debuffs, so it generally falls to others to finish off runners and deliver killing blows. Watch the main fight and save a bit of mana or your combo points for a few instant casts: Frost Nova, fast large Direct Damage nukes, Kidney Shots or Eviscerates, etc.
• Casters: Standing outside melee range helps the Warrior to maintain Aggro. Characters standing out of melee range will not draw aggro until they exceed the threat level of the mob's current target (which should be the tank) by 30%. Inside melee range, characters draw aggro when they exceed the tank's threat level by only 10%. Make a warrior's job easier (and increase your chances of survival) by backing up a few steps before you unleash your fury.
• Warrior tanks build threat slowly when compared to paladin tanks. Because of this, you must avoid attacking until they have sufficient threat. This is particularly important in encounters where mobs are immune to taunt.