Basics of Warrior Tanking

Most importantly, always tank using Defensive Stance with a shield and one-handed weapon equipped. Occasionally you may find yourself needing to switch stances to use an ability in a different stance – the most common example is to switch to Berserker Stance in order to use Berserker Rage and gain Fear immunity, although there are many others – but 99% of the time you spend tanking you will have a shield equipped and be in Defensive Stance. (Unless, of course, you're far above the recommended level for an instance, or are ridiculously overgeared.)
Your job as a tank is not to damage things; rather it is to prevent others from getting hurt. Take aggro onto yourself quickly and keep it there. However, one of the best ways to generate threat is by doing a lot of damage; have a good tanking weapon.
Spend some time and money before an instance run acquiring the best armor and shield you can find (and afford) - you'll be taking a beating and armor helps your healer help you. Never use leather or cloth items, no matter how good the bonuses, and after you reach level 40 switch all your tanking gear to plate as soon as you can afford it (a good idea is to search around for Plate BEFORE you're level 40; not only will you be more relaxed (as opposed to a " since I'm 40, I need Plate Armor NOW" attitude), but you can get a feel for what kind of prices you'll be dealing with when you go shopping for Plate).
Be aware of which special abilities generate large amounts of threat quickly, including Shield Slam, Devastate/Sunder Armor, Revenge, and Heroic Strike. Know how to use each of these effectively, and their limitations (for example, Sunder Armor stacks up to five times on a single target and continues to generate threat past five applications, but it can be made useless by a Rogue using the Expose Armor ability).
When tanking boss mobs, use Shield Block and Revenge whenever possible. Assuming you have aggro, Shield Block will almost always light up Revenge soon after it's activated. Time the use of these two abilities, alternating them to allow for using Revenge every time it cools down. Since [Shield Block] isn't on the GCD keep it up as much as possible while using your threat rotation. (Example: Shield Slam, Revenge, Devastate/Sunder, Devastate/Sunder while hitting Shield Block, whenever it is available.)
A successful Shield Bash prevents casters from spellcasting and forcing them into melee. Practice the timing needed to get this right while soloing.
Most of your high-aggro moves will be disabled if you are disarmed. Listen for the "shing!" noise of that happening, and start off your being disarmed with a Shield Slam (or Shield Bash), which causes decent threat on its own and doesn't require a weapon. Follow up with Battle Shout and spam Demoralizing Shout until you get your weapon back.
You cannot Charge in Defensive Stance, so if you're pulling, use a ranged weapon and lure them back to your group. Alternatively, you can Charge and then switch to Defensive Stance to start a fight with more Rage, but be extremely careful that you don't aggro more mobs than you intended to if you take this approach, and switch back to Defensive Stance as soon as you've used that initial Rage boost. (Use Bloodrage for a Rage boost while pulling with a ranged weapon, or before a Charge to generate even more Rage before combat begins.)
If a Hunter, Rogue, or Mage is pulling, you can Charge, switch to Defensive Stance, and use Demoralizing Shout to cause aggro on all enemies to gather them up, without causing damage that might break sheeps. If charge is not appropriate, use Bloodrage to get the rage for the shout.
Taunt forces a mob to target you for a short time and permanently brings you up to the level of threat of the mob's current target. Immediately after a Taunt, use a threat-generating ability to cement your aggro. In extreme cases (e.g., a higher-level Mage or Rogue), it can be better to stockpile rage until you do lose aggro, then taunt and unload to try and hold aggro longer.
- Taunt has only a minor effect on an enemy that is already targeting you, it will force the mob to keep targeting you regardless of threat for the duration of the taunt debuff; however, it will not raise your threat at all.
Mocking Blow, like Taunt, is temporary aggro only, but without the threat-equalization component – and is not usable in Defensive Stance. If you use it, use the time it gives you to swiftly generate real threat using Devastate, Shield Slam, Revenge, etc. The same applies to Challenging Shout (sort of an AOE Taunt). Unless additional threat is generated during these durations, the taunted enemies will return to their original target afterward.
Only dump Rage if you are confident you have all aggro under control. Otherwise, keep your threat generating rotation up (Shield Slam, Revenge, Devastate/Sunder Armor, Devastate/Sunder Armor.) For multi-mob tanking insert [Thunder Clap] at the start moving the enemies away from any sheeped enemies if necessary and then replace a Devastate/Sunder Armor spot in the rotation with Thunder Clap or mouse-over Sunder Armor.
- One good quick Ragedump in a hectic situation is: Berserker Stance, Berserker Rage, Whirlwind, Defensive Stance. This causes a satisfying amount of AOE damage, and generates extra rage to counter the Stance Swapping. Works best with Tactical Mastery Talent (3/3), allowing 25 Rage carry-through.
- Charge followed by Demoralizing Shout, Thunder Clap, Defensive Stance is a good way to get all the enemy mobs to focus on you right off the bat. Generally works best to Charge the moment your party's crowd control (if any) goes off, Demoralizing Shout to immediately get attention, then make sure you dragged everyone far enough back to NOT break the crowd controlled mobs before Thunder Clap. If the crowd is particularly large, it's a good idea to follow up with the previous ragedump Berserker Stance, Berserker Rage, Whirlwind, Defensive Stance to solidify that much more aggro on everyone before starting to throw sunders. Don't be afraid to ask your party to hold off on all DPS for the first few seconds while you do this.
- In the event of a VERY large crowd, (the Lyceum in Blackrock Depths comes to mind) tanks will want to be alternating Thunder Clap and Whirlwind, and while both are on cooldown jump to Defensive Stance and spam Battle Shout. This gets a small amount of aggro on pretty much everyone around you -- usually just long enough for the artillery to AoE them down.
- Experiment to find other effective pressure release combos, but always finish with a move back to Defensive Stance. Don't spend too long in other Stances, as this puts the rest of the group at increased risk.
As a tank in higher-level instances, you will be "punted" (knockback) fairly often. This reduces your threat, takes you out of the fight for a moment, and can cause falling damage as well. It is possible to switch to Berserker Stance and intercept out of the air, potentially avoiding falling damage and getting you back into the fight a couple of seconds sooner. Remember to switch back to Defensive Stance and Taunt afterwards. If you have a full Rage bar, it may be better to just stay in Defensive Stance and take the damage rather than lose the Rage you've built up.
Main tanks can pop Battle Stance just long enough to Execute a target near death (following up by returning to Defensive Stance and throwing Shield Slam . Indeed if he's being beaten on particularly hard, his Rage bar will spike the moment he stance switches and the Execute will do a particularly large amount of damage, and may well finish off the mob. On bosses in particular, repeated high-Rage Executes can out-threat your threat moves! Use discretion though; if you're having trouble holding aggro, keep your focus on holding it and only do this in situations well under control.
- If there is a Warrior off-tank present however, it's best to let him do this while the main tank focuses on keeping aggro.

Secondary/Off Tank

If no shield, use Battle Stance and a two-handed weapon, or Berserker Stance and two fast one-handers. (A shield is sometimes unecessary in this role, but ask your healer which they would prefer you use.) Also, a Slam rotation with a slow two-handed weapon is good for both dealing damage and avoiding stealing aggro (since it doesn't generate as much hate as Heroic Strike).
Avoid high-threat-generating abilities, and instead concentrate on other Warrior attacks: Rend, Hamstring, etc.
Take on the Shouting for the both of you: Battle Shout and Demoralizing Shout. Keeping these going frees up the Main Tank to concentrate on generating threat. If you're both 70, work out with the Main Tank who's going to keep Commanding Shout up.
Thunder Clap can help, but watch your spacing, avoid drawing new enemies into the fray with it, and avoid breaking crowd control with it. This is an ability that causes extra threat, so the Main Tank may prefer to use it.
Be aware that without being attacked, you will generate Rage at a much slower rate; make best use of Bloodrage to compensate. Charge is also a good quick burst of free Rage, but be careful using it that you don't trigger fresh enemies. Rage Potions can help as well - collect Sharp Claws, Large Fangs and Gromsblood for them.
Act as personal bodyguard for the healer. Be ready to switch to 'Tank Mode' if any mobs get through, and the real Main Tank can't get to them in time. Switch to Defensive Stance and use Taunt until the tank can get there, then switch back again once the situation is under control. (If you're in Battle Stance, Mocking Blow can help in an emergency, but a stance swap is more reliable).
Execute mobs that are about to die, as many times as it takes. (The Main Tank is certainly capable of doing this as well, but his focus should be on aggro management and shouldn't divide his attention unless the situation is well under control).