How do I get rid of some extra belly fat?

I excersize at least a couple of times a week for a least thirty minutes, and I eat a healthy diet. However, I still have extra fat over my ab muscles. (Which are very hard, if I do say so myself.) I know that I can’t lose all the fat, I just want to lose a healthy amount. When I excersize, I use the elliptical machine (or bikes), I do crunches on and off the ball, and I use weights to tone my arms. Any suggestions to get rid of that little extra belly fat?