Some good tips here, and I second the ab-wheel. One of the few fitness 'gimmicks' that actually make a difference (if used correctly/safely of course).
As far a I'm aware, the best thing you can do for a visible six-pack is to achieve a really low (and possibly unhealthy) body fat percentage.
or you can just be really young :) I am in my 40's now and diet really has more to do with what I look like than anything else does. I really like the ab-rollers as well, particularly those ones that have the spring mechanism in it that makes the roll up easier as this is the part where you are most likely to engage your lower back.
I also like how I can get a "deep burn" on just 12 rollouts whereas I would have to do a lot of situps to achieve the same thing. When I was doing situps I always felt it in my back more than my back but I just assumed that was part of it. Turns out it is part of it and I'm glad I stopped before I did something really bad to myself.