This rule is two-fold and applies to all weddings, not just destination. Study weddings before you choose a venue, hire your wedding vendors and purchase services and products. Reason being- you need to know what is that you want in order to know what to ask for. Just take a couple weeks to flip through magazines, read wedding blogs, and wedding books before you pick up the phone. I'll use wedding cake as a simple example. You have to know that there are frosted wedding cakes and fondant covered wedding cakes in order to narrow down which bakers to consider because so many bakers only do one or the other. This is a good thing by the way because that means they are focused on being really good at that art. This leads me to the next piece; select vendors who have a single focus. A photographer who offers video and DJ service? No. This is hands down the quickest way to get less than quality something. Here's what I can guarantee you. You don't want less than quality anything. Nor do you deserve it.
Read rule 1 here and rule 2 here.
Need some good blogs to research? Try Style Me Pretty, Elizabeth Anne Designs, Bon Bon Blog, Once Wed, the Bride's Guide, and Polka Dot Bride.