REP SCHEME VARIABILITY Use different reps, sets and rest to build muscle. Not only will it keep your training fresh, but it will force your muscles to adapt, resulting in building more muscle. MIX
HEAVY AND LIGHTER WEIGHTS Similar to above, make sure that you lift both heavy and lighter weights. Using heavy weights is great to help build strength, but doing lower weight, higher rep movements helps increase the amount of volume you are lifting. This helps build muscles in different methods which will benefit you in the long run.
VARY INTENSITY It is very difficult to lift weights with 100% effort and exertion every time. Your body can benefit from moderating how hard you are pushing yourself. Sometimes you need to go as hard as you can and other times you need to go at a moderate pace. This can apply to both the weights and conditioning, mix it up!