You need to mix a little of the tanner in with the foundation. One of the top choices for ballroom tanner is Aeryjo. It used to be Pro Tan but it isn't anymore. When you do your tanner you have to start with an exfoliating session. When you are done dry off really well and don't have any aspirations of putting your tanner on yourself. You will end up looking silly.
You need to have someone you trust put it on for you. The magic is not rubbing it in. You put some on and gently going one direction apply the stuff. If you rub it in you will streak. It is going to take about a half hour so put it on the night before...that way you won't ruin your dress. I have a few friends who cut it with lotion and that helps it not to streak but you have to put more on.
I would also suggest that you invest in an air brush makeup set. You can match the foundation and it hides blemishes as well as being sweat proof.