Good question, seeing as I am a very new PE teacher.
I hated PE because I was dyspraxic and pretty much sucked at everything. Of course, this was the 70s/80s when there was no such thing as dyspraxia and I was just thought of as clumsy and useless. I had the whole stigma of being last picked for any team game (or second to last if the special needs girl was there).
I found my own way to exercise, initially with jogging and then weights once I got to uni. Now, irony of ironies, I'm working as a PE teacher in a girls primary school. I had no experience when I started (I was qualified as a gym instructor) so it's been a huge learning curve. I actually started taking gymnastics classes myself so that I could teach the girls and realised that I'm not quite as bloody useless as I thought I was. I try to pass that on to the girls - I encourage them to do what they can and keep a positive attitude. Not always easy though because young girls can get very down on themselves.
If anyone does have any good tips for making stuff fun, though, I'd gladly hear them.