It really comes down to a strict diet although I hate to use that term, and a rigorous work out regime that really kicks your butt into high gear and strips the fat and builds the muscle. A common mistake is that people lay off the weights and only focus on the cardio when trying to loose body fat, but in actuality you should really be doing both as the more muscle the build the more your body needs to feed the growing body and so in turn it will help you loose the fat you have by literally feeding it to your growing muscles for energy.
As for myself I am doing weights for a times a week, with doing a lot of spin classes when I can fit them in to really rev up my metabolism and keep it burning. Spin is great for getting your heart rate up and burning calories and thus fat as it is interval training and so it really keeps your temperature up and heart rate going. Another class you should look to try is either a boxingg / aerobic style class along the lines of tai bo that not only gives you a great cardio workout but also a good muscle work out as well.
On a side note, another added bonus when doing spin it really helps build and bulk up your legs and especially your bum if that is one area you want to add size and shape to as well.