You have to force your muscles to get stronger during the fatigue factor of your training. The best way to do this is the utilize dumbbell style lifting Like Mike said.
Take chest for example, there are main muscles at play, but then there are secondary muscles, stabilizers and so on. One trick is to do dumbbell bench, but keep the one dumbbell extended while the other side does its rep, then it holds at the top while the otherside does its rep. By doing this your focusing all types of muscles to keep that weight stable.
You can really do a lot of other movements with the same idea. On things like cable work on your last rep when the weight want to go back down force yourself to not let it go down. Like bicep curls, after your last contraction on your way down hold it at the half way point and dont give in to the weight wanting to continue down.
Diet is another thing you have to look at very closely, supplements, foods, hydration all play a factor.