As Manofkent mentioned, the 'starve your way to nice abs' method will work, however only if you have gotten to a particular level in your workout.
You gotta remember, to see your abs, you need to be on the low, low end of body fat. I'm talking 10%-15% maximum, unless you have huge muscles there already.
Many bodybuilders have actually used a diuretic to cut the amount of water in their system a couple of days before shows. One SERIOUS word of warning - there have been more than one bodybuilder who has passed out, and one that I know of who have DIED from abusing this method. You need water in your system, and this is not the smartest way to go. The only reason I mention this is that if you only want to look good for a picture, you can do this responsibly. If you are only trying to look good on the beach, DO NOT do this - you are already going to be in the sun, exerting energy, and you will want the water in your system or you will have adverse effects.
I am aware that the bodybuilders who did this for shows and had adverse effects were abusing the diuretic, doubling, tripling (or more) the recommended dosage.