I'm planning on seeing a SP that specializes in strap-on training. To those of you ladies out there that provide this service, what do I need to do to prepare for my first time?  Any info will be greatly appreciated!

....use an enema before the visit....

Trust me on that one.

Figure out what kind of mount the strap on has.  Get yourself a dildo that fits that mount in a size that you feel comfortable with.  Play with yourself to become familiar with the size and feel before you have someone else do the deed to you.  Blowfish is a good place to shop.