I try to pack on as much weight as I possibly can, I used to do 12x3..I found myself getting tired quickly, which wouldn't allow me to progress very far. Now I usually do 6-8x3..the first set being medium intensity (12 slow reps), the second one being higher (8 reps), and the last one being highest (6 reps). Example:
Bench Press
Set 1: 135 lbsx12
Set 2: 155 lbs x8
Set 3: 165 lbsx6
I was reading a number of articles that suggest rack lockouts, which allow you to lift much more than you would be able to on the bench.