Just a few other tips on the subject:
* Biggest one in terms of saving money is to fly out of/into Municipal in Belize City rather than International. Fares on either Maya Island Air or Tropic are about one-half to San Pedro compared with fares from International. The savings are less to most other destinations but can still add up. If you need to catch an international flight, you'll pay US$20 for a taxicab (for the cab, not per person) to transfer between the airports, but if you're traveling in a party of three or more your savings still will be substantial.
* The new owners of Maya Island Air have invested some big bucks in equipment and facilities. For example the new Maya terminals at Muncipal and Placencia are big, new and nicer and cooler than the Tropic terminals. They've also added some new equipment -- mostly 14-seater Cessnas, same as Tropic flies.
* Besides Gaz's excellent discount, some other coupon books such as that done by Budget RentaCar in Belize City will also get you 10% off any local fare (but you may have to pay cash or traveller's cheque rather than with credit card). That's a significant savings if you are traveling in a group.