Most gyms I've ever been to offer one day trial for free
Smart ones will say, "Drop in is xx dollars and if you decide to get a membership we'll credit your drop in fee against the membership cost"
The two gyms I go to use that (and each charge $20 drop-in). Otherwise guests always try to use the "I'm thinking of getting a membership but just want to try it out" excuse. But sometimes if they're in a good mood they just waive it too.
BTW Nasser never has to pay drop in for any of the gyms we went to in San Diego (and I didn't either, for showing up with Nasser)