The real key to doing ab exercises, that I have found over the years, is to do them slowly and carefully (well, that goes for any exercise, but I’ve noticed that people have a tendency to bang out ab exercises like crunches and such as quickly as they possibly can) and, this might sound silly, to really focus on the area that you’re trying to work.
What I mean by focus is to really concentrate in your mind on that particular area of the body: what you do is you focus completely on how that area (your abs, in this case) feels and how hard it feels like it’s being worked, and then you make all the little adjustments you need to work that area as hard as possible (relax all other muscles so your abs have to do all the work, etc.)
If you really concentrate on it and do this slowly with good form, your abs will BURN after a proper workout, try doing the same workout quickly without concentrating and then do it my way, you’ll see that it’s like night and day, huge difference.
Now, I’ve got an expert here who’s going to show you precisely how to correctly do a series of ab exercises with an exercise ball (aka Swiss ball), check it out:
Working the Core
Leg Raises (works the lower abs)
Crunches with an exercise ball