I just did this quick 23 min HIIT routine, which focused on the abs. Trust me, by the end of these 23 min you will be sweating!
Warm up: 2 min jump rope
Then alternate 50 seconds of hard work with 10 seconds of rest for three rounds of:
After your three rounds, end with a plank hold (on hands or elbows) for 50 seconds, rest for 10 seconds, for three rounds.
Total work out time: 23 minutes
HIIT (or high intensity interval training) is a form of training that prevents your body from adjusting to a routine since you are alternating the pace and type of exercise you do throughout your work out. Also it helps you to burn more calories longer post-workout. I recommend doing HIIT two to three times a week for about 20-30 minutes. Really you can alternate all types of exercises, just remember to give 100% of your effort during the work interval for maximum results.