I've found that the only time I lose aggro is if
a) I'm wearing crap gear so I can't hold it to begin with
b) DPS vastly outgear me in a dungeon without level sync spamming AOEs (i.e. ilvl 90 bard vs my ilvl68 tank)
c) I zone out and forget to do a flash every few seconds/rotate between targets

A can be solved with gear upgrades, B you can't help other than just upgrading gear again, and C well just comes from practice. Good luck!