Be prepared to have a hard sell pushed on you! You get taken to a cubicle, and when I say they measure you, they don't "believe" in using a measuring tape, they look at you in the bra you are wearing, and go off of that, despite the fact that it could be a completely different style, or could have shrunk in the wash over time etc. they then come back with a variety of bras they think will suit you (will try get you to buy) and then you feel like you have to buy one.
Couple of pointers: they hook the bra on the last row of hooks, and they pull the straps tight. So if you don't like wearing your bra like that then don't go there because that's how they fit it. The only good thing I learnt from them was to lean forward and jiggle about and shuffle the underwiring about so it was comfier.