i found that if you're trying to increase muscle mass, that you should reach muscle failure within 8-12 reps. if you are doing 12 reps or over, you need to use heavier weights. 8 or under, weights are too heavy...adjust accordingly. it's worked wonders for my arms.
also, nutrition and sleep are huge...you need as much of them as possible...also, it's best to give the muscle group a 24 hour resting period until you work on it again.