Don't forget to work the low back muscles when working the abs. Back curls are good, dead lifts are great. Also do the obliques with side bends and a dumb bell, start real light (5-10 lbs.) for 3-5 sets of 8.
In judo, we teach that all strength is transferred through the abdomen. When you push, you push with the whole body. If the abs/back are weak, the effect of the push is lessened.