I asked this before and didn't get a lot feedback, so thought I'd try again -- how often do you change your workout routine and how do you go about setting up a new one?

I know you should change routines every few weeks because your muscles become accustomed to an exercise and plateau. I admit I'm lazy about changing, but don't think it's affected me adversely. I've been using the same two routines (a Mon/Weds routine for cutting, and a Saturday routine for building mass) for 21 months now. I hit a plateau a few months ago but recently must have pushed past it and getting good results. But I wonder if I changed up if I could get even better results. I just don't know what I would do to replace my current exercises (mainly dumbbells for the cutting and machines for mass).