I never had that trouble much, though I always tended to get serious with the opposite sex pretty quickly, and being rather a staunch monogamist I more or less took it as read that if I had a girlfriend then I wasn't supposed to flirt with others, so I didn't. I suppose it's fair enough if no claims have been agreed, but as soon as I got interested in one girl then she'd be the one I was talking to and any others quickly paled into insignificance until such time as it was clear that it wasn't going to work with the current one. Otherwise things would get complicated.
I'm not saying it's the best way, just that it's the way that came most natural to me. When it comes to who you can flirt with and who you can't, I guess the only useful rule is that whatever freedom one person exercises, it's OK for the other person to do the same. Personally I don't object to jealousy in people as long as it's not excessive and doesn't turn nasty.