As someone who enjoys playing both tank and healer (PLD and SCH actually) I find the best thing a tank can do is look at his DPS. If he has two dps who do very little AOE damage, the tank shouldn't be pulling large groups as this will prevent the healer from dealing damage and the dps are still gonna burn everything down one enemy at a time.

If there are DPS who can deal massive aoe DPS (smn, rdm, brd, etc) the tank needs to pull large groups and use his mitigation tool to the best of their abilities.

When I'm pulling large groups as a PLD, not only am I using my CDs, I'm throwing Clemency on myself because I find that to be a very good part of my mitigation kit. There's also knowing when to NOT pull more, for instance the Halgai in Bardam's Mettle (that first pull) and the pull with the eagle because that eagle, along with the Dharas are going to EAT that tank alive, I don't care how good they think they are, unless they have Hallowed/Holmgang/Living Dead ready. Scratch Holmgang, that is a momentary death prevention.

As far as the healer side, know your CDs and learn to use them. The tank does not owe you the easiest dungeon possible, so be ready to use Largesse and use it often. You should familiarize yourself with any other healing CDs your particular job may have, like Lightspeed for AST and Fey Illumination for SCH. Your actual off global cooldown heal CDs should also be used extensively.

Know your kit, do the best you can in any situation.