The most important thing is that the girl likes to suck cock: not all WGs do. If she does, then you will feel it and enjoy it. Personally I dislike the use of a hand on the shaft during the head bobbing as I think it is cheating. The other thing that works is for the girl to start with very little pressure (or suck, if you will) but then gradually increase the contact sensation. This gives the impression that she is really into it and, well, one just has to produce the goods after that.