6 packs are mainly to do with diet. You can have a huge 6 pack but it can be covered by flab. Now I know you said that you are not flabby but I thought I would just point that out for any others looking at this too.
Exercise wise what are you doing at the moment? Any sport, what sport?
The abdominals respond to differing exercises, like for example I wasn't doing many ab exercises targeting my lower abs so I had a really well defined 4 pack but then introduced some lower ab exercises and I am already seeing results.
I've been using the NAVY SEALS workout and the following are the ab exercises:
First you need to warm up - main ab exercises follow (these are not the warm up exercises):
cross arm sit ups
half sit ups
cruncher sit ups
atomic sit ups
good morning darlings
Try to leave out any rest time inbetween exercises as any rest will decrease the gains you could make. I would recommend starting off with 5-10 reps per exercise as it can get quite intense.
Don't overcook it - first time you try it take it easy and if you don't know what the exercises are just ask or google them