Mid airs happen. At my local club we reduce its occurance by one simple rule: trainees fly far. In other words, if you are a trainee, you fly further up and further away. Another way is everyone flies anti-clockwise (wind direction is left to right). You do whatever aerobatics you want into the wind and fly circuit the other way, so if you mess up in the aeribatic move you dont hurt anyone. If you are experienced enough to fly closer and do more aerobatics, you give each other ample space and fly the maneuver, fly out of the box and let someone else try. And if the hot-shots or jets take to the air, everyone either keeps their distance or lands.

Inspite of all this, we have had mid-airs, on one day we had three followed by two hairy-scary landings from people including me who got disturbed by the mid-airs. In fact I landed too close and then caught the plane while it was doing a good land speed with the left wing and turned it around thrice before it stopped (throttle cut did not work). Needless to say after that everyone landed, packed up and left for the day.

We also had a few bird hits on a few other days

Ameyam