After taking this measurement, add four to five inches to your underbust measurement, which is your band size.
For example, if you are a 33 underbust, you might want to try bras with a 38 band size.
If it's an underbust corset pattern you're looking for and you're fairly comfortable adapting patterns to your own needs, try the Underbust Corset Pattern Drafting Instructions on ultharkitty's LiveJournal page.
My boyfriend is a crossdresser too, and my underbust corsets fit him perfectly and give him a nice defined waistline.
Once you have that measurement, record it and subtract it from the underbust measurement + 4.
Run a tape measure horizontally around your underbust--that is, under your bust and around the back.
Underbust Corset - Just as the name implies, the underbust corset begins just under the bust line and extends to the hips (and in certain cases, beyond).