As I sit here, in the coffee shop, I'm watching a guy furtively glancing at attractive women who are walking by. He's objectively okay looking, but his body language is sending all sorts of mixed and unintelligible messages. Arms crossed, legs crossed with that nervous wiggle. He looks with his eyes, not with his head. His posture is slouched, he just doesn't exude any confidence at all. From the guy perspective, if you had to fight any guy in the room, he'd be the on you'd pick first. Not that he's a small guy, he just looks weak and uncertain. Women take one look at him, and subconsciously put him into a category: "he's never going to approach me." Women want to be approached. They want a guy who appears confident -- she can wait until later to crack that shell.

So here's my recommendations for this guy, maybe it applies to you too:

(1) Eye contact -- making confident eye contact involves your entire head, face, and eyes. You turn your face square to the person you're looking at. You smile slightly, with your eyes and your mouth. If she reacts positively by smiling, then you're free to approach.

(2) Body posture: Sitting up straight and powerfully gives the impression of fitness and care. You're not some lazy scrub. Shoulders square, legs apart, arms uncrossed. Think about how a president or business leader sits at summit meetings / major conferences, etc. These guys exude confidence, and it's all planned and trained (for the most part). You have to practice sitting well and projecting your image.

(3) Appearance/dress: It's not as critical as you might think. As long as you're clean and tidy looking, then you're fine. Basically you're trying to avoid sending up red flags. Wearing a TapOut shirt is usually a mistake. Clean cut, clean shaven or light shadow, a playful / light demeanor, unafraid to wear your emotion on your sleeve.
Sit next to other people, in the middle of the room. Corner sitters look weird and it's hard to approach from corners anyway.

The main point here is to read up on body language and the message that various body / facial postures sends to others. It's an incredibly powerful subconscious method to help establish attraction. A girl won't mind rejecting a confident guy because she knows he can handle it like a man. A weak/beta guy with a weak approach will lead to soft rejections, which are "ambiguous" to the clueless guy.