Some small tips I just recently offered a friend:
Trunk twists: take a broomhandle and place it across your shoulders. With back and neck straight, facing forward, twist from the waist to one side then the other. You can do hundreds of these before feeling really tired. They work the oblique muscles that run along either side of your waist, helping to tighten them and create a trimmer appearance.
Stomach vacuums: tense the muscles of your abdomen and then suck your stomach in as much as you can while still breathing normally. This is a trick commonly used by professional bodybuilders to help emphasize their torso and make their waists appear slimmer-works just a well for cosplay photos. Hold for as long as you can, release, and repeat. It strengthens the abdominal muscles and helps tighten the waist a bit.
Above all else, maintain proper posture at all times-the only way to prevent the stomach from becoming distended is to keep your back straight and avoid slouching.
Freedom is the right of all sentient beings-Optimus Prime