I'd never look at you sideways, but I could see myself looking upside down on you in a mutually agreeable position....
^^^ see? I'm flirting with you.
My default mode is "flirt"....I flirt when I don't have to, it's second nature to me.
Lots of people respond positively to that, some will roll their eyes but it's all fun.
If you can vaguely sexualize something (and get away with it)...that's flirting.
Imagine you're a personal trainer, and you tell a guy "wow that's quite a pump you have there..." *pause* "I suspect I'm not the first woman who has told you that though" *look innocent, devilish smile* (guy subsequently signs up for a year of additional training)
Flirting isn't a requirement for a GFE, but it's the icing on a cake. It makes for happy repeat customers, particularly if you can personalize it.
Just find something....ANYTHING.... in common. "oh, here comes my favorite USMC tattoo!" Guys like compliments (women do too, but I digress). "I've never asked a guy this before but wow! what cologne are you wearing?" (they tell you) "That's quite a panty dropper!"