I used to havea six pack, about 3 years ago. Then I got fat.
The key, I found, was to hold your abs in when you are doing exercise. This means they are always working.
The best method for this, I found, was swimming. If you swim a casual breats stroke for about 30 minutes, and hold your abs in during that time you will get out feeling as though you have done a million sit ups.
Something else which contributed massively was someting I used to do in my football training. Probably not a great deal of use to anyone, though. Unless you happen to be a goalkeeper in your spare time!
Basically, I used to sit on the floor with someone stood up throwing a ball to my left and right. I'd rol to the side and catch it, then throw the ball back with the momentum of sitting up, not using my arms, if that makes sense. So the whoel sideways movement, back and forth, was done using my abs. As I came up the idea was to move myself forwards about a yard, so I had to effectively use my abs to lift and move me. It was really hard work, left a big mud stain on your backside and made you feel a bit sick. I'd tend to start in the goal mouth and finsih around the half way line. It was a difficult exercise, but a really enjoyable after feeling.
There are literally hundreds of theories around about the optimum way to tone your abs, though. Just thins month in Mens Health there are 5 PT's who all give a different exercise as the best one.
It's worth having a look at their website, as they do give you lots of exercises that aid the building of abs.
It's also worth noting that they tend not to be visible until you have less than 10% (I think) body fat.
