M.B.
I'm a parenting writer working on a sleep book, and a mom.
We went through the very same thing, we co-slept until about 7 -8 months. What I've found is that just about every expert agrees that the most important thing for parents is to be committed to consistency. The uncertainty of crying it out one night then picking up and rocking the next, or putting her down asleep one night and awake another, or putting her in bed sometimes but not other times, is what creates the endless crying.
One ped. I interviewed said: "give me a weekend-- start on Friday and she'll be sleeping by Monday" and that's a sensible plan-- it gives you a chunk of time to be committed to the new routine, plus you don't want to have to worry about having to go to work on the days after the two toughest nights!
So Friday night, do your soothing routine (feed, wash, song, etc) and put her down when you see her eyelids start to get heavy but while she's still awake. It's going to be a loud night, but keep in mind you're not hurting her, you're helping her learn a valuable life skill, even if she's disagreeing with you and doesn't care to learn it! Then wait at least 15 minutes to a 1/2 hour before you go in, and when you go in, DO NOT pick her up-- then or for at least a full six hours (which is how long she should be able to go between feedings if she's average-sized) unless she's got a seriously dirty diaper (if that's the case, change it quickly and get out, no cuddling, all business). Avoid eye contact, which is stimulating. Just show yourself briefly to let her know you're there. If it sounds like the crying is winding down, don't go in at all, because she'll just get riled up again. Keep it up and by Sunday night she really will be settling much faster. The big thing is not to feel guilty-- you are not hurting her, endangering her, damaging her sense of trust or any of that. She doesn't like sleeping alone (my 6-year-old still doesn't), but it won't hurt her.
Also, not to ramble on forever here, babies seem to do well with early bedtimes. If they get over-tired it makes it tougher. Most moms I've interviewed say a bedtime of 7-7:30, with a feeding at about 1, works best.
Good luck....!