I happened upon a post on this board about Elegant Adventures and contacted them and got airfare for a little over $100 less for the exact same flight I was looking at out of Miami through them. Granted I have never used them before but I checked it out and it seems legit. You might try them. The website is www.elegantadventures.com.
They seem to specialize in Central America and have some bulk airfares.
Other than that I can only suggest signing up for the fare watcher through Travelocity and Orbitz and/or see if you can get a better airfare out of another airport that you wouldn't mind driving to. I've driven 200 miles to another airport simply because it was worth the difference in price.