I would be very reluctant to tell a client what he is doing is "wrong" unless he asks, made it clear he was trying to improve his skills, my satisfaction was imprtant to me or he is hurting me. Why? because I believe that it is about them enjoying themselves.
I have never told a client he is doing something wrong nor suggested so. It is about conveying not only verbally, but by body language, wiggling to slightly different angles.
Example, when a guy is almost getting you there, but carrying on the same will achieve no more, saying "Oh goodness keep that up with a slight bit more pressure and you'll have me squirting" means you squirt, he is seriously feeling like 'The Man', everyone is happy!
They will enjoy their booking more and leave feeling great, no need to be negative or patronising. In my experience it has bought the gents back through the door, even those who tell me they NEVER see the same WG twice.