This isn’t really answerable in a forum discussion, as it all varies too much depending on circumstance.

I guess the basic idea is to make someone feel good and wanted without going overboard and coming across as any sort of creepy. This is a fairly fine line, though, and where it is fluctuates wildly depending on the person, situation and expectations of the moment. You’re also juggling body language and tone in addition to your words, so really anything can be made flirty, or go overboard, all depending on recipient/mood, delivery and circumstance/timing.

The first thing I’d probably start thinking about is how to identify the times and individuals where any flirting will be welcomed, which is also going to vary quite a lot. Dates are a pretty safe place to start, for obvious reasons.