we have a policy that if the rest of the team feels you havn't done enough, you can't go. Now with everyone paid their £150 flight ($300 ish) deposit, we will try and refund anyone who we deem can't go. I believe we have a letter that we show any member doing nothing, that we threatent o send to their parents, explaining the situation. Usually that does the trick tbh. Last year we had no policy and got stuck with 2 useless people going just for hte fun of it and doing absolutely nothing all year.... and they would admit if you asked them...
We also keep note of certain tasks that we ask people to do , to provide evidence and to keep a track. Such as fundraisers etc... if you dont turn up, you get a mark on your name in the black book of doom.
One of our kind sponsers gave us airmiles for medium class tickets (ie not cattle class

) on the plane, and the 5 most dedicated are getting them .