To keep it simple, pick a muscle or few that you want to work in a day, then use several different exercises for that.
Say chest/tricep day:
Bench press, dumbbell bench press, pushups, inclined bench press, cable pulldowns, etc.
GREAT INFORMATIVE SITE (has pictures for each exercise, and descriptions):
http://bodybuilding.com/fun/exercises.htm
If you can think of an exercise, it is in this site.