Lying on the floor doing a bunch of leg lifts isn't the most effective (or efficient) way to slim down your hips and thighs. Instead, get multiple muscle groups firing at once and burn up more calories with this Roundhouse Kick
Stair Drill instead.
To do it: Stand on your left leg with your left foot slightly turned out, both elbows bent, hands in fists by your chin.
Bring your right knee up into a 'chamber' position by lifting it to hip height and pulling your heel in and behind you (toward your butt), creating a straight line from your hip to your knee joint.
From here, extend your right leg out with your toes pointed, as if you were tapping something with your shin. Bend your knee back into chamber position and then lower your foot to the floor, only lightly tapping it down (don't put weight on it). Repeat again, but this time, extend from the chamber two times before lowering your leg. Repeat again with three extensions before lowering.
Continue repeating until you have worked all the way up to 10 extensions in a row, and then reverse the stair drill on the other side.
Options: If you need help with your balance, stand next to a wall or lightly hold onto the back of a chair (about 2 ft. away from it). Be sure only to use the wall or chair for light support; Don't start to lean on it when you get tired! Keep your abs in tight and your body upright as you do each drill.