50% in advance + (when travel is involved) proof of ROUND TRIP fair and acceptable accommodations on the other end, etc. Deposit is refundable (less 35%) within 30 days of making the booking but will not be refunded is less than 45-days away from scheduled date. The only exception would be "Act of God" which does show up every now and then thanks to weather or in one instance a major Earthquake and in another, the hotel burnt to the ground...

(I really think I jinx things...)
With Act of God we encourage rescheduling and in most cases, will offer an extreme discount on the balance due or donate it in the name of the client to a charitable cause related to said incident. This has always proven a potent selling point.
Even with fundraising programs INSIST ON A DECENT DEPOSIT! Such groups are notorious for canceling and not letting you know. I insist on $300.00--$500.00 no matter what, which is held against ticket sales (which gives them incentive to sell seats vs. sit around thinking a few flyers and signs will bring the people in.
Anywho... that's how I used to deal with it all.