The trick isn't adding weight to your abdominal excercises. Things like crunches or sit ups wont give you the definition you need. If you're serious about MMA training, you need to do isometric abominal excercises, because they give you the torque necessary to strengthen holds like Triangle chokes on the ground. Coincidentally, these are the exercises that promote definition of abdominals. I have 10% body fat at 180 lbs, and I have clearly defined abs. Do things like planks, side planks, "saturday night" pushups (that's when you go down on a push up and simultaneously bring your right knee to your right elbow, and then do the same on the next pushup with your left side). You will begin to see results in 2 to 3 weeks. For planks, hold the position for a full 60 seconds, flexing your abs and gluts.