zero eye contact (or very little) or any kind of acknowledgment that I am there, no smiling, folded arms, head bowed down, eyes looking at the floor, ear or headphones on, checking her phone, etc.
i usually look for more open inviting kind of body language, lots of glances, long eye contact, smiling etc. so if you look like you don't want to be approached, I will respect that, and not do it. I might say hi or a hello just incase. :)
For me it's when a girl obviously invests what I would consider to be way too much time and effort into her outward appearance. It shows a high degree of vanity which, in my experience, almost always leads to a better-than-you attitude.
body language, facial expression etc. I don't get guys coming up to me when I'm alone, and my family and friends say it's because I'm always frowning and in deep thought, so I always look serious and too caught up in my own thoughts. but when I'm with friends, guys come up to me more often, because I'm always smiling and laughing and being goofy. so yeah, I'd have to say body language, facial expression and all those stuff has a lot to do with whether guys want to approach you or not.
I look like I'm pissed off all the time or I wear clothes that make guys stay away from me. I really don't like attention like that. Whenever I end up dressing up and a guy does hit on me I just talk to them very awkwardly and look at them like...why on earth are you talking to me? lol.