On thing that has really helped me gain size is keeping the workouts new. You are probably aware of the fact that your body can and will adapt to basically any program that you can put it through. Let's say I was going to do a set of Pull Down's, I would do four sets, the first set to 10-12 reps, second 8-10 reps, third 15 reps, fourth 4-6 reps. This style of training will work your fast and slow twitch muscle fibers. It will also give you a great pump. I incorporate this style of training on most body parts. Try it, by the end of your work out you'll be a wreck.