Admittedly, I am a flirt. I especially do it at the the deli counter, which tends to be busy. I figure that if the women customers aren't very friendly (I think less so than the men, on average), the least I can do is try to brighten the day for the ladies behind the counter with a little harmless flirtation. If I think they might not be so receptive (e,g., the young girls don't want an older guy flirting with them), I either don't do it or do something fairly safe. You can usually tell a woman that her hair looks nice without her being offended (whereas if you comment on her clothes she can assume you are talking about what is underneath the clothes). Complimenting a mother's children is also safe . . such as, "Your children are so well-behaved". They may reply, "Well you should see how they act most of the time!", but I still think they like hearing it.
When I go to shop for women's clothing, I always flirt with the women staff. Women love men's admission of ignorance -- "I need to get something for my female supervisor that she can wear to work but still be comfortable. I have no idea what that would be." The lady will ask me how my boss usually dresses, what colors she likes, etc. Then (if she is good), she will show me various combinations that she likes and see what I think. Complimenting a woman selections (rather than saying, "I like what you have on", which again can be too strong) is always good. Even if you don't like, don't say "no, not that" or "nah". I like to take my time anyway, but taking the time to make a good decision like this shows any woman that you do care and are thoughtful. And I NEVER speak critically about ANY women to the sales lady . . . saying, "Well she wants to look feminine, but she is big so forget about that." would get you dirty looks, even if the sales lady is slim. I always say things like, "I want to give her something that will make her feel pretty" . . women love hearing guys say stuff like that. After the purchase, I always thank the women profusely for all their time and help and say something like, "I know she will love it. Thank you so much." Some might call this common courtesy rather than flirting, but I never say stuff like this to male sales staff, who don't really go out of their way for male customers and don't want men being too emotional around them. If they help me, so what? . . that's what they're there for!