Here's something you could try.
The problem a lot of 'guy friends' get into is they start to become too complementary. They start to treat their interest too nicely, do favors for her, put up with annoying conversations, etc.
If you're already in the friend zone, it's going to be really hard for you to get out. You're probably better off just starting over.
If you want her to start liking you as a love interest, instead of as a friend, you have to stop being 'friendly' with her.
How to do this:
1. Date other women (doesn't have to be seriously). Talk to her about the women you're dating, and how they're all really cool, attractive girls and how you're really into them. Don't complain about them; that will only make things worse.
Of course, you might find someone else you really like while you're working on this step; in which case just forget about your friend and move on.
2. Tease her, A LOT. In a fun way, of course. If you can make fun of a girl in a way that makes her laugh, it'll really turn up the attraction she feels towards you.
3. Stop doing things she asks you to do, unless you actually want to do them. Don't do her any favors, don't take her out to eat, don't put up with her drama. If she starts talking about her jerk boyfriend, excuse yourself. If she asks you to go to a party with her, and you know you're not going to enjoy it, refuse. If she wants you to take her car to get washed, refuse. If she wants you to write a poem for her, refuse (or write her a hilarious poem that makes fun of her in a subtle way). Remember, you don't want to be her 'friend' anymore, so there's no reason for you to do 'friend' things for her anymore.
If she's going out to eat somewhere you actually want to go, then it's okay to say yes (but don't be too eager about it).
4. DO NOT TELL HER HOW YOU FEEL ABOUT HER! Don't tell her you love her, or that she's the 'only one for you', or anything like that. Save it for when you're proposing to her. Seriously, there is no faster way to ruin your chances with a woman, because you're giving all your power away. If you already love her, she doesn't have to put any effort into the relationship, and that's boring. Relax, kick back, and let her PURSUE YOU.
In fact, if you start getting out of the friendship frame, and she starts being more flirtatious towards you, start busting her balls about being in love with YOU. Give her crap about annoying things she does, and how she's screwing up her chances with you, etc.
5. Don't worry about whether it will work or not. If you're freaking out about whether this is going to happen for you or not, you're almost guaranteed to screw it up. If you really want to change your relationship with this girl, you have to be prepared that you might not be able to be friends anymore. Get over it; always remember - there ARE plenty of fish in the sea.
It won't be easy, and there's only a small chance you'll get the result you want. Like I said, it's almost not worth trying.
