I wore clothes easy to get on and off. I did spend most of the time in my undies (and no bra) when I wasn't wearing a dress! It was a bit confronting at first, but I figured the assistants had seen heaps of people before in all shapes and sizes. Most of the shops I went into had shoes for you to wear, and most of the time I had to stand on a box!
I'd also suggest wearing no makeup - you'll have dresses over your head many times and you'll end up a mess (and probably have to wear a cloth over your head/face to protect the dresses if you do go in made up.