Make sure you got Threaten on all the time. If you still have problems you can invest some skillpoints to get Taunt (active skill) that'll force enemies to focus on your tank for atleast a couple secs. But you shouldn't "really" need Taunt, aggro in DA is a bit strange, it's based on the armor type, massive armor will generate more aggro. (and dmg offcourse)
So if you have a party with 3/4 members in heavy armor/massive armor aggro could go all over the place. If you only have one heavy armor wearing melee character with threaten on it "should" be easier to hold.
There are also items witch reduce aggro (rings/amulets with -15% etc) throughout the game, give those to your mages/healers/rogue.