Since I primarily use workout DVD's as my source for working out, I change my workouts up regularly so my body doesn't build tolerance to just one or two workouts and plateau. Variety keeps me still interested and I don't get bored easily
Change both cardio and strength workouts. Change your variety of workout equipment as well. I find that by changing my workouts more often, that I'm pushing myself harder to workout. More effort put in, means more calories burned and the more calories burned, means the more weight that will be lost.