4. Be mindful of comfort as well as inclement weather. Rain may wreck havoc with hairstyles, sound equipment, musical instruments and the officiant's text thereby sending everyone inside, but that's not the only thing to consider when planning an outdoor wedding. Keep your comfort and your guests' comfort in mind. If the temperature falls below 50 or exceeds 80 you really should consider using your alternative indoor venue. Don't put the men in three piece suits or black tuxedos if the weather is expected to be hot. Position the ceremony in a shady spot on hot days. While the bride may only be at the ceremony for 15-20 minutes, the guests can be sitting out there waiting for it to begin for half an hour or more. Choosing a shady location also makes for better photographs as it is pretty much impossible to remove squinting with photo editing software.
5. If it is in your budget, hire professionals to handle the decorations for an outdoor wedding rather than doing it yourself. Wedding planning companies and some florists can provide packages that include chairs, flowers, arches and other such structures to enhance the location and provide a visual focal point for your wedding. The advantage here is that should weather become an issue, professionals can more easily tear down and reassemble your decorations than you could. In their capable hands, you won't have to scramble at the last minute and break a sweat. Additionally, they have the experience to know what kind of decorations hold up to the elements better than others. I've seen a lot of do-it-yourself efforts turn out wonderfully, but unfortunately, I've also seen too many fall flat - once quite literally.
6. Consider an indoor venue with access to a beautiful natural setting for your portraits instead. You can always get married inside and then take advantage of the beauty and romance of nature during the post-ceremony picture taking. Here in Florida, many beachfront hotels have ballrooms and restaurants with glass walls or huge windows overlooking the beach. You can get married in the comfort of air conditioning without any risk of wind or rain ruining your decorations (and hair!) and then step out onto the beach for spectacular portraits. Likewise, public parks and gardens will often have an event facility enabling you to get married in comfort and photographed in nature.
Lest I sound too negative regarding outdoor weddings, I can assure you I have officiated oodles of successful weddings in natural settings. I would estimate at least 75% of the weddings I officiate take place outside and by far, most are perfectly fine. It's the exceptions that stick in the memory and all of these would have been helped tremendously with the considerations above.
Be realistic and flexible and you'll do just fine!