Yeah, what Lizzy said...
I do basic crunches, reverse crunches, double crunches (basic and reverse at the same time), bicycle crunches, and plank twists; that's all I can remember right now. I am working on side planks but I can only do about ten in a row because they are really hard for me. Pushups and plank stuff (like plank rows) also work your core. I have used Pilates too and did notice a difference. I am finally seeing some abs so it is worth the pain

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You can probably get pics on how to do any you're not familiar with from Google, or from an exercise book.