Planks are pretty good. I'm a huge fan of crunches. If you have a balance ball, crunches on a balance ball will target your abs better than traditional crunches (will also help work stabilizer muscles). My favorite lower ab exercise is to lay face down over a balance ball. Walk yourself out with your hands until the balance ball is under your shins. Roll the balance ball in and out with your legs (the motion will resemble a reverse crunch except that you're face down- you're basically bringing your knees into your stomach).

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