That sounds really annoying, all I can think of then is to try running every now and again to cement that idea in your sub-conscious. Trying to get over the whole body tensing up thing sucks, It use too happen to me a lot if I had to run away or throw a punch. My muscles contract and I get incredibly slow, sluggish and weak. I found flying away easier at first since it's the first thing i'd always do when going lucid till I've more or less mastered it (Still have trouble carrying people in my dreams, get weighed down and slowly fall back down to the ground).
I sometimes fly in my dream so I dont particulary have a desire to do so when lucid dreaming. The one thing I do when I am not testing in lucid dream is... well... errr... (>.>) I guess to make it softer for some people I'll just say it this way. I enjoy some impossible fantasies.
For some unknown reason despite my rather perverted personality. I can literraly count on one hand just how many normal(non-lucid) dreams I had that involved erotic theme.. Lucid dreaming is trully fun in that aspect.