Tanning is an important part of looking the part for ballroom dance. Getting a proper airbrush tan helps you look thinner and more muscular, and accents your graceful arm and leg movements, especially in rhythm and latin dances.
First, let's talk color. You definitely want to look tan, but you don't want to look like a figure model or a beefy weight lifter. You want to look elegant and healthy. You want the tan to accent your body, not distract everyone from your technique. Competition level color is for figure models and bodybuilder competitions, not ballroom dancing. Keep that in mind when you are being sprayed from head to toe.
Second, if you are renting a dress, you need to get your tanning done at least 24 hours before you wear the dress. The spray tanning process generally involves using a bronzer initially, so that it is easy to see where the tanning solution has been applied and ensure an even application. This will rub off on your clothes (wear something black or that you don't care about getting bronzer on permanently), your sheets, everything your body touches, until you shower. And you need to wait at least 8 hours in most cases to shower to let the tanning solution work it's magic.
If you wear a competition dress prior to showering, you have the potential to get bronzer all over the dress, and when mixed with sweat and tanning solution, this creates a stain that is near impossible to remove, ruining the dress, or making it undesirable for others to wear. Simply showering will prevent this from happening! Get your tanning done with plenty of time prior to your event! This will also help make sure your tan is even and not a blotchy mess. You can also help extend your tan by moisturizing often, especially if you are dancing on multiple days.
Also, we all know that being in the sun too long can lead to skin cancer and premature aging, so I cannot recommend a spray tan highly enough! There are some excellent spray tanning companies out there and they continue to improve in their technique and their tanning solutions. Some of the best that I have found use warm air and can actually spray lotion at the end to help lock in your tan. The 'fake bake' looks more natural than ever and keeps you from damaging your skin. Just make sure that you get it done in a timely fashion so that you and your dress can both continue to look amazing!