Ok, instead of offering the useless "Just do it" statement, I'll see if i can be of more assistance.
I was just like you only a few months ago. I've been working on this stuff for about a year now, but it's picking up momentum lately, having gotten the number of a complete stranger today. So heed my words.
The problem is, you're approaching them like you want to marry them. You might not be thinking about this consciously (because that would be weird and lame), but in the back of your mind, the thought process is:
Boobs! -> Get Number -> Marriage.
Which is sad. The key thing i've learnt in the last few months is to just talk to them. They're human after all, and they speak your language. If they turn you away, so? They obviously aren't the kind of person you want to know anyway, so don't sweat it.
Also, no matter what you say, be confident in saying it. Even if she laughs and makes a funny look because what you said might not have been quite right (we all make mistakes in conversation; a bad pun here, a failed joke there), but just laugh it off. If you have to, claim she doesn't have a sense of humour.
Also, you mentioned the "Hi" "How are you?" "Good." "........." pattern. This is more common than you think. The important thing is to ask questions. Not like in "The 40 Year Old Virgin", where you say nothing but questions, but just simple things to keep the conversation flowing.
All in all, the main thing to remember is, there is absolutely no risk taken when you say Hi. No risk at all. Guys think there is risk involved; getting turned away. I see getting turned away as a filtering process. I'll make a mental note not to approach similar looking people again (demeanor wise; ie, perhaps they work at the same place, and carry themselves in a similar manner). It's very judgemental of me, but it takes the risk out of the whole activity.
Now i'll say it: Just do it. Girls aren't robots, they don't speak when spoken to exclusively. If you say hi, then the volleys begin, a short pause might give her a chance to ask something. Which shows interest. Go from there.
Syn.
edit: hahahahaha at the previous post. Have fun.