I agree with the comment about my comfort zone being in a dress or a nice skirt, with a little jewellery and perhaps a touch of makeup.
My big "pushing the boundary" was the first time I went to any sort of gathering as female. I was just getting past the nervous introductions when a colleague walked in and recognised me. That was the most terrifying point on my journey, it's all been getting better since! He's gay by the way, which was why he was at an LGBT gathering, and he certainly knows to keep his mouth shut.
After that, going into a department store as male and booking a bra fitting was a breeze.
My experience was that a smile and a polite request is very well accepted. The lady at the lingerie counter just treated me as a customer who was likely to spend some money with them and found me a booking.