To quote Dougster: Just ask.
On Tuesday night I was at an Indianapolis dive bar where lap dances are 2-for-$20 and a bucket of four bottles of beer costs $11 (I think). On several previous visits I've been satisfied with a two-dance grind resulting in a happy ending (what I call a "lapgasm" -- in my opinion they're much too common to name after someone's TUSCL handle -- nothing personal, LapDanceKing!).
This time I chose a somewhat older dancer with huge natural breasts and, when we got to the not-so-private lap dance area, asked what kind of fun we could have back there. She asked what I had in mind, I answered that I was hoping for a blowjob, and, after a brief negotiation, she proceeded to give me a very discreet but enjoyable CBJ -- in a club not known for extras!
Just ask. Every stripper has heard it before, many will say no, but some will say yes.
Many other members BESIDES Dougster have offered this advice.