I try to either bump up the weight or reps a little each time. Lets just say to make things easy your doing curls with 50lbs on a barbell and you can do 5 reps. Next week do one of two things, either shoot for 6 reps or slap a 2.5lb weight (or whatever the lightest weight you have is) and do your 5 reps. The key to gaining stregnth and building up your muscles is to lift more than normal and to change up your exercises every so often, if you dont change them up your body will get used to them and you wont see gains as quickly.