I've been lurking for a few days, saw this thread and decided to give my 2 cents....
Last winter, right after Christmas, I managed to hook up with a girl who I had been friends with for about 5 years. Years before we hooked up I told her I liked her and whatnot, but it obviously didn't work and it was definitely AFC on my part... I could literally go on for hours about all the details and things that occurred leading up to it over the years, but I'll add what I believe were the key factors.
1. She knew I was sexually attracted to her. All girls are different so I would imagine each one would require a little different approach to letting her know this. I didn't want to create any awkwardness between us so I made sure to keep it somewhat subliminal, not enough to destroy the friendship if she wasn't interested, but enough to make the thought cross her mind more than once.
2. Others have said this and I believe it to be quite important - she MUST be comfortable alone with you. Before we hooked up maybe 4 or 5 times we went to her house and watched movies in her room, alone, in the dark, on her bed. Every now and then one of her female friends would join us, and I made sure to make them laugh and have a good time. Having her friends comfortable with you can't hurt.
3. Appearance. I'm no supermodel, but I made sure to dress sexy, shave clean, cologne etc the last few times before we hooked up. Movies in the bedroom aren't exactly a social gathering, so that adds to letting her know you're plans because in her mind there's a good chance shes probably thinking you got all done up for her. I never told her i did however, I wanted to keep the curiosity going.
4. Persistence. After you've succeeded in letting her know you want to fvck, and she still wants to hangout, by all means hang out with her. Chances are its because she has caught on and wants to fvck as well. This may not be true in all cases but in mine it was.
5. Don't over-talk her. I'm sure there are better words for this but if you start hitting her up more and more and answering texts/calls the second you get them she might think you're starting to actually have feelings other than sexual ones. Be friendly, but don't be "nice".
6. You gotta make a move when the times right. I didn't just straight up say, "lets ****", I built up to the moment over time. The night we hooked up, we were both laying on her bed watching a movie after some hot chocolate at duncan. She got up to go to the bathroom and I slid over towards her side of the bed a little bit, and put the covers over my feet. When she returned she got back under the covers, then I sort of nudged her foot with mine, I forget what i said but reallllly slowly i started rubbing her toes with mine. I left it at that, leaving her to make the next move. Within minutes she just straight up took her shirt off, then her bra, and I went to town.
I know this isn't worthy of a sticky or anything, but I figured i would add my experience and what worked for me as a means of advice for others, because it is definitely possible to hook up with friends of the opposite sex.