I concur with the other posters, discuss it with a TA who is EXPERIENCED with destination weddings... Also, I recommend doing your own search because some travel agents may attempt to steer you towards a resort that they receive a benefit from. I know my TA (love her) but she tried to hard to get me to go with El Dorado resorts because of the incentives she would receive, but the cost was just ridiculous to me, especially because of the $800 outside vendor fee!
First, think about what location you might really want to have your wedding. Think about what you really want in a resort, large or intimate, if you have any preferences for the room (mine was the in-suite jacuzzi and the shower), look at restaurant choices, if you want to be at a resort with children/or if you will leave right after your wedding to go to a different resort for your honeymoon, and definitely look at the options for wedding packages and think about how you envision your wedding being.
You may be able to get resort prices for next April, just not air, and that could give you a pretty good estimate for guests. But most likely, 9 months before your wedding is when you'll be able to make your reservations for air & resort so I would jump on it at that point because prices do flucuate quite often.
ummm, I can't think of anything else right now, but if you have a question feel free to ask.