Depending on the site, most token sites use pay-per-minute method but for private shows via the site itself. In between being tipped for acts, remind viewers you are available for private shows and just make sure to set the amount of tokens you want to accept for viewers to even be able to begin a private show. (You don't want someone to only "last" 1-5min unless your token-per-min is set super high. The way I see it, if you are going to pull me away from my public room audience, essentially risking loss of attention aka traffic per SECOND I am not present in front of camera, then don't bother taking me to private show at all.) I am quite transparent about my logic when explaining this to viewers who want to insist to start a pvt show for a very low amount or very low amount of time. They usually understand and due to my politeness, return when they have enough tokens to spend on me. ;) So never be afraid to stick to how you want things to be set up to maximize profits for yourself.
Also, make sure to set how many tokens per minute you want to receive during the show. Personalize those settings and then remind your viewers that what you "really" want is to be taken to private, that way you can try to maximize the amount of time spent stuck streaming. LOL If you are NOT looking to do privates at the time and focus on group effort of public chat/viewers to reach goals then definitely say that too so people know how to use their tokens best per stream.