Here's that question

In short it's tough for a guy to get a girl to switch over from thinking of him as a friend who is a boy into a bonefide boyfriend...
Your best bet is to errm hedge your bets - think of it this way, you're not saying never - you're just accepting that today she is just a friend...Start treating her that way...Start the dating game elsewhere, hunt out interesting alternatives...
Ration your time together - be too busy for her because you are dating or at least doing something else...
One of two things will happen; (1) you'll find someone who imeediately sees you as a sex-god is really the right girl - rather than the one who can't see the wood for the trees - or - (2) She'll say wtf, why aren't you my best buddy anymore - to which you can honestly answer, you're a great friend - but I need some initimacy, so I'm spending time with say 'Jessica Alba' ;) thus you are in demand, and perversely more attractive...
This may or may not work, but the worse that can happen is that you move on...

Yeah, since you say.."she knows that I like her," and she hasn't made it clear that she likes you, then it sounds like she wants you as a friend rather than a boyfriend. at least for now. ..
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that's not great news for you, but it can change. ..
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as rudecherub suggested, you should start trying to date other girls. ..
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that way a couple of good things could happen -- either you'll find someone who you really like and continue your friendship with the girl you're writing about; or the girl you're writing about will start to get a little jealous, see you as potential boyfriend material, and decide she wants to be more than friends. ..
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however, you don't want to start dating someone else just to make your friend jealous. that would be a pretty nasty thing to do to the person you start dating...

It may sound harsh, but in my experience, the "ladder theory" is true in almost all real-life cases: http://www.pixton.com/comic/izecq10p -- once you're a friend, you're a friend. And even if you improve to a "friend with benefits" you'll never become a couple, let alone have a successful relationship...
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My suggestion is to keep that friend as one of your best friends ever, go out with her, have fun -- but get into a relationship with somebody else...

Hmmm. Well, it could work out and become a really awesome, wonderful relationship. Or, she could just not be attracted to you. Take her to dinner. Ask her how she feels about the two of you dating. She'll tell you if she isn't. I had a guy friend who I looooooooved talking to. We hung out all the time, went to dinner all the time. We had a lot of fun together. One night, he suggested we make out. I said no. I said I just didn't feel any fire, or sparks between us. He said he wanted to stay the night because my sheets were a lot cleaner than his. I said no. He said, "Can I at least see your boobies?" I said no. We remained close friends. He got married to another girl a year later -- I still think he's great. Just ask her what she thinks about you guys dating. If she's not attracted to you, she'll say no. If she is attracted to you -- happy ever after. You can still be friends either way...

You're going to have to make a decision. If you go out with her, then break-up, there's a big chance that your friendship will die. Is it worth the risk? You need to decide that...