Go to he pub. Hee Hee.

Seriously. Sit ups, crunches etc combined with a good diet and plenty of cardio work. The key is consistency tho. Keep it up regularly and you'll be fine. Unless you want body building type ab's then i can't say as i've never done that. Some people do heaps of ab work and it doesnt look so good even though it is tough. Umm..... Say I look at Sam Soliman and Sakio Bika from the contender they both have a very high level of commitment and work heavilly on the ab's. Bika looks different from Soliman though. Bika is more defined and may be due to body type. I guess it depends what you are after.