I think it's generally understood that you should change up your work out routine every few weeks to couple of months because your muscles become too familiar with an exercise and they start to grow at a slower rate. This is basically what P90X is build around, the concept of keeping your muscles guessing, but in a shorter timeframe.

I'm horrible at changing my routines. I've been using the same routines (one set of exercises for Mon/Weds and another for Saturday) for the last year and a half. The funny thing is I must have pushed past a plateau, because I've been getting a number of compliments lately.

How often do you change your routine, if at all? And more importantly, how do you decide what exercises to replace them with? Do you use a different exercise to target the same muscle but a different part? Do you give that muscle a break? What's your approach to setting up a whole new routine?