Leslie, it makes sense to 'mix things up' after a while. The best way to grow and to tax your muscles is to change what you're doing. There are many different variables involved that you can play with; number of reps & sets, amount of rest between sets, whether you pyramid or not (or whether you pyramid up or down). There are also other techniques that can be used, instead of going to positive failure, you can go to negative failure. You can use strip / drop sets, static contractions and the list goes on

I had a discussion on a few of the different techniques in my gym log last week with Kate. You can read it here. Keep in mind that many of these techniques are designed to push your muscles past the point of failure and can be dangerous if you're new to weight lifting. This is why the BFL program is a good one if you're just starting out.

Cheers!
Nat