You probably already know how a spray tan works: A mist containing dihydroxyacetone (DHA) is sprayed onto the body, either in a tanning booth or with a hand-sprayer. The DHA reacts with dead skin cells and dyes them brown. This may not sound very romantic, but the results can be lovely -- especially if you're worried about looking pasty in your wedding gown.
Keep in mind that if you're fair-skinned, you should start with the lightest spray tan shade that's offered. Otherwise, you may look more like the Great Pumpkin than a sexy bride. After your spray tan application, keep your skin moisturized with lotion (this will keep the newly darkened top layer in place a bit longer) and plan to re-spray about once a week.
It's a good idea to do a test tan about two weeks before your big day. That way, you'll know how it looks on your skin. And, because you need to forgo bathing for several hours after your tan is applied, don't do your touch-up the night before your wedding. Schedule your appointment 48 hours in advance.