I usually select random and base the way I'm going to fight on the fighter's strengths and weaknesses.
For example, GOD FORBID I end up using Drew McFedries, this guy sucks on the ground. Dog****. He's a pretty good striker though, I'm going to try to use my sprawl and clinch reversals to keep it standing.
Now, if I get somebody like Georges St. Pierre or BJ Penn, it's game over.