Ill add in a set of higher reps for each muscle group from time to time at the end ie chest ill do one set light weight at the end of my heavy sets. Mostly though its best to change out an excercise every 3 months or so for each of the excercises you do in all your muscle groups. For instance instead of bench switch to dumbells for a month; then go back to bench and do incliine with dumbells for awhile etc: do this with back, tris etc. Keep changing up your excercises. to confuse the muscle and make it work.
Thanks,
-steve