I found this great image on Charmed Valerie on how to flatteringly wear leggings. She also has some great advice, which I've posted below the image.
"What to wear with leggings
- Something that covers your hiney
- Long shirts that end a little bit below your hips/upper-mid thigh
- Over-sized sweaters
- If you really want to look sophisticated, wear a bootie/ankle boot/boots that leave no room for skin to peak out.
- An extra layer (like a long cardigan, sweater, or vest) looks even better but the initial visible layer should cover your crotch area
- Balance clothing proportions by NOT wearing something tight-fitting with your leggings."