Crunches, Cardio and Time.

You gotta remember Britney Spears does work out alot and have a personal trainer, but shes also dancing countless hours everyday which helps alot too. Also nix the sit-ups and do crunches. While not fully bad sit ups so strain you and dont work as much of the ab as people thing. Crunches do and you should only be able to do 20 in a set and feeling it to be doing it right. If you can do 30+ no stop and not feel anything somethings not right.
I have heard though from various people that belly dancing classes are a great way to work the abs also, since alot of the move concentrate on that area. You can get a video lessons too if you dont have time for the class. :-)