Flutter kicks, baby! We did these all the damn time in basic training and AIT, most of the time as punishment, but I lost five dress sizes in four months!
Down to a 6/7 because of those damn things...
Lie down on the ground (or mat, if you have one). Put your hands under your ass, palms facing the ground. You do this to protect your tailbone and to keep you from using your arms to "cheat". Lift both legs 6" off of the ground and start kicking! Keep your knees locked and do NOT let your feet touch the ground!
Looks kind of like
THIS.
You only have to do this for a few minutes every day. It'll hurt like a bitch at first, but it gets results!!