Cos heaps of people fuck this up....
I take it you are talking about your six pack, as most blokes do....
the following advice is spot on...
paterson3713 - Abs are the same as any other muscle. You should be treating them the same. Use weight if needed to get the same effect.
Kimmykim - remember when you're on the exertion mode of the rep to try and pull your navel in towards your spine so that you get the most shape out of the 6 pack.
paterson is spot on - train with the same intensity and days off that you would say your biceps
kimmy is also spot on about sucking in your belly button (using the Transverse Abdominus) - this also takes your External and Internal Obliques (they help twist the torso side to side - ie twists with a medicine ball) out of the exercise, concentrating effort on your six pack (Rectus Abdominis - and yes it is only one muscle not six), and also helps protect your lower back on say situps (very important - sit ups can be murder on your lower back)
Get nice abs
Remember, you still wont see any six pack if you are a fat bastard....so
diet also becomes part of the routine....
good luck....
btw - i hate working abs and hamstrings.....