agree on a rate "for time only" in pre-meet communication......presumably, you've already ascertained what the two of you will "do" in a professional, polite, discreet way
with new-to-me providers, I then ask that money not be mentioned again before or at the meeting.....I tell them that I'll pay at the end of the meet and ask if that's ok......
of course, sticking with well-regarded, "known" providers mentioned here in the forum or over at daddysreviews is one sure-fire way to avoid a sting