There is such a phrase now as a "thigh gap"? I have one, but it's because I'm bow-legged, which I always thought was undesirable. Well, if THAT's in fashion, can we bring back fair skin, too? Then we can stop having to look at all these fake, orange tans when summer comes back around.
Thigh gaps have to do with the shape of your pelvic bones, either you are born with that shape or you aren't. It's not something you can get through diet or exercise (other than completely starving yourself which isn't healthy in any way). Most of the time, when you see magazine images with one, it's because it's been airbrushed that way, or the person is extremely underweight.
Beyonce is probably the most beautiful person in the universe and she does not have a thigh gap! She works out all the time! You are either born with one, or you aren't. I was 85 lbs and 5'5" at 15 years old and I didnt even have a thigh gap. So don't even bother with it, seriously.