I have a hard time completing a workout because of 3 kids. Constant interruptions, them watching me, talking to me, wanting something, being noisy, arguing...you name it.
I recently purchased Tony Horton's 10-minute workouts on DVD. His P90X workouts are just too much for me personally. I really like his 10-minute workouts because I am a lot less likely to walk away from it. If I am into a 30-minute workout and get tired, I get discouraged by the fact that I still have 22 minutes to go...but at about 8 minutes of a fairly aggressive workout, if I am pooped, it is much easier to complete the workout when I know there are only 2 minutes left. Then...If I want to do another of his 10-minute workouts, I go for it. If I don't then I have the satisfaction of knowing I did not quit in the middle.