The truth of the matter is that that may likely happen no matter what you do, the same way a heterosexual person is bound to flirt with someone who may not be interested in them, regardless of their orientation.
The simple fact of the matter is that at least a handful of times in our lives, for whatever reason, we're going to be interested in people who aren't interested in us the same ways, and there's just no avoiding that. Just because we're queer doesn't mean that other queer folk are going to be automatically interested in us; essentially, we take the same chances regardless.
There are no rules for flirting with men or women; that really depends on who you're dealing with, where you're at when you're flirting and the like. Best bet in my book? Friends first. In other words, you can start to get to know someone you're sexually or romantically interested in the same way you'd get to know anyone else: you say hello, you introduce yourself, you strike up conversation and find out about them, you take it where it goes.
As may also be obvious, if you're hanging out in GLBT circles, at a queer event, using queer personals ads, etc. you can narrow the field a little more easily.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson