We've done our reading, and we've had enough partners to know what we like, what we don't, and what limits we want to have in place. Sometimes we'll spontaneously come up with a variant, or read about one that still falls within our limits. For years we were swingers, and learned a lot of fun variations on things we already liked; new people often do old things in new (to us) ways. Some of the best new variations required an extra person, though, so those won't be achievable for most!