Well, won't hurt to look at some anatomical references (skeleton and musculature, etc.)- I usually start with drawing the femur, then building muscle mass (or just drawing a quick, elongated oval) on top of that. Generally speaking, women's legs tend to have more fat on the inner thigh and hips than men's, thinner ankles, and less-defined calves and knee-caps.
I did a quick Google and found this link for ya- ain't my style, and may not be yours, but at least it's professional. I'm sure if you looked around a bit more you'd find somethin' useful online.
Another tip, though it might sound weird, is if you have a girlfriend who understands being an artist, ask if you can study the shape of her legs and feel them. Strangely enough, I learned a lot from giving my ex calf rubs after she would finish daily soccer practice. Being able to hold something in your hands can really help you get a feel for how it looks in shape. Plus, looking at a 2D model never quite gives you as good of a feel for depth and volume.