It's everyone's job to make sure they are not grabbing aggro from the tank. There are little bars for each party member showing his/her enmity. Players need to keep an eye on the Threat Meter to make sure they're not pulling aggro.

Let the tank solo a little bit before healing. Especially at low levels, the tank should not be on the verge of death after just a couple of hits. (If he is, he can use Foresight and/or Bloodbath.) Especially at the beginning of fights, don't keep the tank's health topped off. Just enough to keep him out of danger.

For longer fights, after his enmity building combo (Heavy Swing --> Skull Sunder), he can use his DPS combo (Heavy Swing --> Maim), which will increase his damage 20%. So when he switches back to Skull Sunder, he's grabbing even more hate because he's also doing more damage.

And Gladiator for Flash (level 8) and Provoke (level 22). Those are huge.