Well, to be quite honest... the way I accomplish that is to basically keep fucking them! Making sure you're keeping in contact, but while still following the law of least effort. It's ok to initiate some contact every couple days via text so long as you have an objective in mind when doing so (such as setting up another meet). If they're not initiating contact (read: texting first) AT ALL... then are probably not FWB material, want more investment from you, or worse- not interested in continuing the relationship.
Bottom line, in my opinion: You're going to have to keep sleeping with them on a regular enough basis to keep them from going into auto-rejection. If you can make time to see a woman once a week or so, and sleep with her on nearly every date/interaction, then she is FWB material and wants that too. These women are actually not hard to find, but you need to come off as completely non-judgmental that she's engaging in this behavior, show your sexual side/vibe as a constant, don't text her to death (law of least effort) and be deliberately moving each interaction toward sex.
I have a girl right now that I see once or sometimes twice a week and everything above has been working to keep things fresh and keep her interested. But, when you find what's working for you- do that. Some of the best strategies and ways I go about things may not be things I learned on this site, but rather just things I picked up on along the way. I noticed it works for me, so I keep doing it!
Good luck!
NarrowJ
