I used to wear contacts until one came out in high wind at the Mickleden Straddle race one year... leading to a long and tricky final descent over rough ground with one eye closed.

I now have a pair of Oakley sunglasses frames with prescription lenses fitted. They've got rubberised side pieces which are also slightly sprung so that they stay fixed to your head. I didn't believe the optician when he said they wouldn't come off, but after jumping around his shop for a while I was convinced. I've had them for 15 years now and they still work really well.