I set up a "Tape Tracker" spreadsheet to keep track of my workouts. I list the title of my workout, the instructor, type and length. I then track the number of times I've done the workout. I also award myself imaginary money so that I can see when I've worked off the cost of a new workout. I find it's a great incentive for me.
Also after I've done a workout 15 times I do it less frequently to prevent boredom.