Situps are a good start. Next, do situps with a medicine ball. I do 50 reps when I'm lazy. When I'm doing a hard workout, I do 100. My goal is to get up to 200. Leg raises tone the lower portion of your abs. When you do the leg raises, try to keep your legs as straight as possible. Also, do them with your hands behind your head (people put them at their side, but that gives better leverage and makes it easier. It's more challenging when they're behind your head). Another good exercise, doing "planks". Go into pushup position, but put your elbows to the floor. Get as low to the gound as possible, and stay on your toes (both feet to the ground). Do this for 1-2 minutes. Lastly, trunk twists while holding a medicine ball. Focus on turning at the stomach, not with the arms.

Those are exercises you can do alone, these you can do with a partner. First, get into situp position, then have your partner kneel onto one knee over you; with their shin across you abs. Have them slowly press more and more weight onto you, until they've got as much weight as they can give (or you can handle). These, I like to do timed. 30 second hard intervals work well. Next is a variant of leg raises. Lay with your back down, with your partner standing up at your head, facing you. Grab their legs with boths hands for leverage (you'll need it). Now do a leg raise. The partner will push your leg down and to the left, then to the right, then to the center. The object is for your legs to go through a sudden change of motion. Lastly, this one will be closest to taking a body shot. Lay down in situp position. For a period of time (1 minute), have your partner drive a medicine ball into your abs. Next, lay on one side and have them drive that medicine ball into one side. Then switch.

Of course, don't do all of these at the same time. Try to do only one to work upper abs, one to work lower, etc. All of these are listed so that you have good variation in your ab-work rountine. I hope I've been able to help you.

Edit: And DD brings up probably the best point of all. To add to that, make sure that you control your breathing. Know what your entire body is doing at all times, that includes your breathing. You may be inhaling when you see a punch coming, which is bad. I know that when I get faked out, I exhale on the fake, then start to breath in. That's pretty bad, since if a body punch is coming after the fake, I'm more likely to get the wind knocked out of me. It's easier to get the wind knocked out of you when you're inhaling.