Wake up your core and kick your abs into high gear with this circuit workout. It should take you around 20 minutes, and the best part is you won't need any equipment at all.
Directions: Warm up with five minutes of light cardio, then repeat each three-exercise circuit three times. Start with 10 reps of each exercise, building up to 15 reps of each move as you get stronger.
Here's a link to a printable version of this workout.