If you were filling the cups with real mammary/ adipose tissue and very sensitive (milk producing) nipples, the fitting of a bra would require far more artwork and science. We don't have the same functional requirements for a bra. For men, this isn't rocket science, and should be no more difficult that finding a good fitting t-shirt.
The advice on this thread is excellent. There's a host of CD sites that "talk you through" bra fitting; and most of it is common sense. The rest is pretty much determining how to fill the cups, the desired size of fake boobs, making cleavage, creating a look. Cosmetics and fashion.
Don't get me wrong. It's possible to make a bad buy out of the gates (too tight maybe?), and end up wearing something uncomfortable. Size it according to your chest circumference as advised.
I got very lucky with my first bra purchase; a common cotton Playtex living bra; 42B. I bought it a couple inches smaller than my chest size. It didn't have any hard wires or parts. It was perfect; so comfortable I wanted to wear it all the time (not possible). And it holds a "load" admirably; from rubber falsies to water balloons.
Part of the fun is the discovery.
