Ok so put your index and middle finger right above your belly button. Push in, breath deep and suck in for ten counts. Release and breath. Repeat this 10 more times. Do at least once in the morning and once at night. More often if you can. Increase the amount of reps from ten to twenty slowly over time.
Another good one, laugh. Sit up straight and chuckle for a few seconds focusing on bring your stomach in. Breath and do again. This is great anytime and anywhere. So next time don't hold that giggle in, it's exercise.