Taaaaa daaaaaaahhhh, here I am!

Wear the best unpadded/simple bra that you own and a flattering top, the fitter will want to assess your shape with your top on.

I have NEVER been measured without a bra ON. They need the girls in place to do the right measurements.

It's very quick and they are very respectful of your sensitivities.
They measure your band size (under the girls).
They measure your chest above the girls.
They measure the shoulder length from back to front.
They measure your bust (all the way around the girls and back at nipple level).
They ask you questions about your lifestyle/wearing/cothing preferences.
They ask you questions about your girls' actual shapes.

You wait in the fitting room while they go fetch you a few options to try on, they wait for you to change, come back in and assess the fit and comfort level.

Here's the trick, ready? When you've settled on the right one for you, buy only ONE.

Go down the mall, find JCPenney and tell the bra broads that you want this bra at a better price. Let them fit you also because VS often wants to send you home with a less practical, sexier version than you need.

They always come back with the same one for me, a Bali soft-cup underwire. Then, I buy a bunch of those, go back to VS and tell them I changed my mind, get my $50-60 back.