Yep measuring their improvement is critical (they dont like to run so they need to see the why) good point Lickety Split... initial improvement is easy as you are correcting fundamental mechanics (running without the hands, stepping on the back side of the base, stopping/slowing before the base is hit, no explosive acceleration, etc...) and they surprise themselves that they can shave half a second or more by just fixing simple things. By seeing their own improvement they get hooked on the drill.. I had girls fighting to go first just so they could try and beat their time (and each other's time... they love competition after all)
Keep doing running drills that requires them to run past the bag. There are many many drills that you can use but the way I see it make them do it so much they get it stuck in there mind to not stop short.