You need to change your routine very often (about every 6 weeks) this include no only the type of exercises but also the type of technique. Continually changing the stress placed on the muscle groups prevents adaptation and forces the body to respond. Utilizing a variety of techniques not only breaks monotony in the gym but allows for periods of growth by alternating tissue breakdown (with overloading, heavy exercises, large resistance) and tissue repair (with rest, lighter activities, and various methods).