If you're doing a 30 or 60 day challenge, simply do any 1 exercise on your rest days. I like Hit the Mitts, but a simple jump rope or jabs exercise will do.
Due to inconsistent save problems which will RUIN your streak, be sure to check your workout time before you start and after you have RESTARTED the game. If the time has increased, then the workout saved. EDIT: 08-2011 This was fixed in a patch and doesn't seem to be a problem any more]
To check the workout time from the main menu:
Player Tracker > Workout Journal > Time Spent - Total.
EDIT: Make a back-up save to a memory card every now and then, You never know when this game will freeze and wipe out your stats [Yes, it just happened to me!]