You need to make her absolutely fiend you, and see you in a way that isn't normal for her. Do things for her.. shit that shows that the relationship you have isn't a normal friendship. Don't be too clingy with her, because it will drive her away.. make her think about you and miss you and shit. you're in a pretty tough position, because bitches are wack as fuck... but nothings impossible.
I just recently went to a wedding and the bride went up and said her speech and she was going on about how when she first met the groom they became friends and they were friends for years... she never saw him as anything other than a friend.. but over time, he was running his game and being a gentleman, and she became attracted to him more and more.. she then realized that she couldn't be away from him..... and BAM dude was smashing >