discovered this by accident, but after some testing was proven to be working 100% of time.
Me: jedi consular, lizard pet from first zone.
Step 1: order pet to attack target (i use ctrl+1) - distance doesnt matter, you just want him to enter "attack mode" basically
Step 2: cast a buff on your pet (all of my tried buffs worked)
Step 3: The mob is now instantly aggroed on you, even if its halfway across the map.
Enjoy