You have to exercise and eat healthy in order not to gain fat. And mantain that, which is the hardest part I guess.
And about the belly-dance, well it's something you learn. She does it with so much ease because she learnt that at 9 years old, so she already has 20 years of practice. It's like a second nature. But with lots of practice and will, you can be good at it too. After all, there are lots of women world wide that can belly dance, even though they aren't famous like Shakira is.