^^^ I've got a couple of Arnie's old bodybuilding posters and he has monstor abs. He certainly wasn't lean at cut like a boxer, but you could chop would on his abs...

I agree that it can be very hard for some people to get a great six pack.
I think having a good six pack and having really strong abs are pretty different. If you can get yourself REALLY lean and your body fat percentage right down, you'll be able to see your abs with barely ANY exercise directly to them. Losing fat is the key to having a good looking six pack (and a little exercise). Strong abs obviously requires work which everyone above has talked about. You don't have to over do them though. If your exercising your abs even just once a week and you can't see them, it's most likely you need to lose weight, not up the ab exercises.

My favourites ab exercise is full stretch crunches with my girlfriend sitting on my stomach