I just started riding fixed a few months ago as well. When I first started, it took a while to remember that I couldn't coast. Just concentrate on that fact until your legs remember. Muscle memory is a great thing. Don't ride a freewheel bike in between. It'll screw the muscles up. And while you're on your fixed, instead of locking your legs to stay in a comfortable position (because you think you have a freewheel), just relax your legs and put your weight on your saddle instead of your legs. They'll keep rotating with the crank. Otherwise you could get bucked and hit the ground.