Best and most effective way to tone abs ?

Title says it all really.

I have been doing sit ups but nothing really seems to be happening.

Thanks,

Try crunches

and a low fat diet maybe?

Sit-ups, crunches, v-sit-ups and leg lifts. Google them as I can't be bothered to explain.

The majority of people do sit-ups wrong. Lay flat on your back and bend/arch your legs, but not all the way so they are tight and closed up, have them in a reasonably relaxed position. Put three fingers from both your hands lightly on both temples on your head leaving your thumbs and little fingers loose. Have your head about six inches off the ground and raise up using your stomach to do the work. Do it fairly slowly so that you properly work your abdominal muscles. Start off with a routine of about 10 and then work your way up to greater figures as your strength increases. This is the only correct way to do a sit-up, no matter how comfortable other ways feel, you will be straining something that you will feel the effects of after a whille of continual incorrectness.