Wait to book until about 3 months out. We booked our April trip last June... but everyone waited until around Dec//Jan to start booking air as it stayed about $900 RT until then (from Atlanta). The sales dont usually hit until about 3-4 months out. Just keep an eye on them each month and they will eventually drop.
Unfortunately Delta doesnt fly to Belize or most all of us would have cashed in some FF miles instead! opefully they'll be flying there by next time we go.