In addition to what the guy above me writes, I find it helpful to pretend that your dpsers are impatient and reckless. You've got a big toolbox full of talents to build threat, take aggro, stun/interrupt, mitigate.
On a pack of 5-6 mobs I would mark skull, charge in, thunderclap, move a little to get the mobs in line for shockwave while I shield slam skull, shockwave, about 1 sec before the AOE stun wears off, pop shield block and start working on building AOE threat faster than the dpsers.
Then I use random rotations depending on cooldowns and procs. Focus is on getting a head start on the main target so I can switch to the off-targets. I do this by putting shield slam, conc blow, rev on the main target (1 of each should be enough), switching to off-targets for a few seconds, then checking back to ensure I have enough threat on the main. To build AOE threat I use shockwave/TC whenever they're off CD, also trying to cleave on every auto-attack if I've got enough rage. Other than that I just tab through all of the mobs and check for threat.
When you lose aggro of a mob, just taunt + shield slam and you have about 2 seconds of aggro, giving you time to single-target nuke that mob to gain 5-15k threat on it or to switch to another target and then back. It's all about priority and having a sense of how much time you have before a dpser passes you in threat.
It's kinda difficult to convey everything in print but basically I have one "AOE-mode" set of rotations and one "Single target" set. AOE-mode is used in the beginning to get every mob on me, and then I continue using it throughout the fight. I use single-target mode to get far ahead of the DPS on that target, then switch when I feel like I have enough to keep threat for a few seconds.
Single target priority: Shield slam > Revenge > Conc blow > Devastate. Heroic strike on every possible auto-swing.
AOE prio: Shockwave > Thunder clap > Demo shout (pretty weak) and Cleave on every possible auto-swing.
With Omen, Recount and basic knowledge of the threat generated by each skill, you can experiment yourself and see what works on what packs. You'll have an easier time keeping mobs glued to you if you do 1.5-2k dps, so increased expertise/hit/strength works for you.
Warrs have awesome utility in their talents and by clever usage of shockwave, conc blow, mocking blow, taunt, shield bash, heroic throw, disarm, you can keep mobs from hurting your DPSers even without having aggro.
Glyphs: Revenge, heroic strike and blocking glyphs will skyrocket your single-target threat.
Finally I can recommend this spec, not saying it's perfect but it's worked for me when I got through all the Wotlk heroics.
http://www.worldofwarcraft.com/info/classes/warrior/talents.html?tal=350200002300000000000000000000302000000000000000000000000053351025000012520330113321 When raiding I would take 1 point from Cruelty and put it in Vigilance, but didn't find it neccessary in heroics. If you have trouble keeping aggro I'd recommend taking vigi, personally I didn't and prioritized the DPS increase from having more deep wounds procs.
Ideally one mob should die within 5-10 seconds or so if dps are doing their job =D