...Lately I've started working on "stomach vacuums" and have really noticed a difference. My abs were starting to get bulgy from weighted ab work in the gym. I've been doing these vacuums about 2 weeks and my waist is smaller already!
To do stomach vacuums, expel all your air and pull your abs in like you're trying to touch your spine with them. Really squeeze the ab muscles. Hold for several seconds and repeat. They're a lot harder than they sound, but they work, and you can do them just about anywhere.