I do crunches and bridges, as Laurel recommended, as well as leg lifts.
Leg lifts: lie on your back with your legs straight and your hands placed palms down under your butt for support. Draw your belly button towards your spine and lift your legs about 6-12 inches off the floor. Keep your legs straight and hold them up as long as you can, rest and repeat (depending on your specific goal).
Crunches: Lie on your back with your hands behind your head, your feet on the floor and your knees bent. Draw your belly button towards your spine and slowly lift your shoulders off the floor to crunch your ribs towards your pelvis. Focus on shortening the distance between your rib cage and pelvis and avoid using your hands to lift your head/shoulders. Do as many as you can, rest and repeat (depending on your specific goal).
(I thought descriptions might be helpful--although I'm not sure what "draw your belly button towards your spine" means. The descriptions are from 24hourfitness.com )
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How about leg lifts?
The descriptions are from 24hourfitness.com 
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