I usually just talk about something very unlikely to sound predatory or sexual. "I like your hair," things like that. Hair is one I bring up frequently also because one can usually tell whether a lot of effort was made; as I mentioned, I think complimenting something that involved effort is more impactful than just random, possibly meaningless things about the person.

For example, I don't usually say anything about clothes (on men or women) unless there's something particularly striking about them; recently I saw a woman wearing a shirt expressing a political opinion that is not popular in this area (but with which I agreed) and complimented her on it because it seemed like it probably took courage to wear. However, I wouldn't comment on someone wearing a plain white tee.

I have learned that I don't necessarily always enunciate properly and therefore should say "I like your shoes" rather than "I like your boots."