Unfortunately, you have picked up an ugly truth.
What city are You flying out of? Charter island preferences?
A few ideas:
1) frequent flyer miles - harder than ever & you may have to sit on the wing
2) Fly middle of the week
3) Pick islands that have direct jet service & are popular with land tourists
ie; St Martin (SXM), St. Thomas (STT)
4) Try various departure cities that You can get to cheaper on another airline ie; Chicago, NYC, Atlanta, CLT
5) Charter Company Airfare - several times during the year Moorings, Sunsail & others offer airfare discounts, at least get a quote to see how it compares
6) Try airline websites vs Orbitz, etc
The bad part about it is that the airlines have no logical fare plan. Their numbers jump all over the place