I know that completely passing is probably never going to happen -- but I do know that sometimes I do better than other times. Here are some things I think make a difference -- I'd be interested in what you have found...
1. Skirts too short. A dead giveaway. I wear short skirts, but never too short if I want to be in public.
2. Brunette rather than blonde. Who doesn't what to be a blonde -- but it screams fake.
3. Perfect eyes and eyebrows. I agonize over eyeliner and over brow shape. It seems to make a difference. I've found that the less eyeshadow the better -- darker and bigger eyes are not good.
4. Conceal the back and shoulders. I am top heavy and know it -- if I really want to blend I wear a cape.
5. Reasonable boobs. Not too large.
6. If I am in a completely straight environment, such as a mall, I'm spotted less often. I think people don't see what they aren't looking for. "Alternative" neighborhoods are impossible to pass in, but then again they don't care how you are dressed anyway.
7. Smiling. I can make most suspicious eyes change mood by flashing a smile. I suspect GG's have known this for years.