Hi I am a Fitness consultant/manager so thought I would give you some advice here.
Like Judimac said you can not spot reduce, unfortunate as that may be!
Sit ups will work some of the ab muscles but will do nothing to burn the fat that covers them, that is all down to healthy diet and a complete and balanced cardio/resistance work out.
Sit ups and cruches still have their place but there are many many different exercises that help to strengthen the whole core area, not just the front abs. Think of your abs as a corset, going all the way round, front, sides and back. Pilates moves are fab for acheiving a balanced core. Do a search on Core stability and you'll find lots of essential info. If you want any more detailed advice give me a shout
