Um, well, try to add some battle-damage and some areas where the paint has chipped off. Also, I find a light drybrush of scorched-brown around the treads makes it look "used" and like it's been driven a bit.
Oh, and here's a pic of my predator to give you an example:
Predator
Oh, and here's a pic of my predator to give you an example:
Predator