The Red Light Bar is next to a half decent, clean brothel. Any taxi driver should know where it is.
Darlings night club, just off Wenceslas Square is/was also quite good. They do a half decent cabaret & you can take your pick of the ladies. Beer is really expensive in there though, so sip it.
Stay away from the street walkers, they will try & rob you. Plus, they're skanky as hell.
Not that I've ever partaken of the delights in any these premises. *cough, cough*