I second AVs advice because that was what worked for us.
Some hints: to eliminate frustration, we dedicated a week of evenings to the idea of 'teach Kiddo to stay in his own bed'. (he's always fallen asleep with one of us with him, so I get it) Knowing that *this* was going to be the activity meant that we treated it like that. I actually camped out in the hallway right next to his door so that the minute he got out of bed and started walking out to us, I could get him. I had my laptop, a book and a few other things to keep me entertained and just hung out all evening; believe it or not, I slept there too for the first night.
The first night was tough; he popped out of bed several times and I just quietly said "bedtime" the first time,then nothing, just kept taking him back to bed. No talking, no smiling, no extra hugs, no interaction other than putting him back in bed. He was even up at 1 a.m. to try getting us into his bed or him into ours. Nope. Back into bed.
The second night, less pop-ups and no late night wake-up. Was able to move to my own bed in the middle of the night. Third night, my husband did his shift and there were only a few times Kiddo popped out of bed. It kept improving.
I do want to mention this, because I think it's important--- with many kids, you will have recurring seasons of what we'd call 'sleep issues'. Kiddo is nearly eight now and we have no silver bullet. Kids do have anxieties which pop up right as the lights get low because they have nothing to distract themselves, or they get it into their heads that they are going to be 'in charge'..... so I would encourage you to keep strong routines around this. Don't be fooled into thinking you will damage her if she cries and cries because she's mad that you are imposing a new expectation on her-- and don't give mixed signals by allowing it some nights and not others; instead, let her know that you believe she can do this. I like the idea of a new snuggly or maybe a special nightlight (this is one many of the families I know liked:
http://www.amazon.com/Cloud-Twilight-Constellation-Turtle...
Also, and this is not to get into a tussle with anyone-- do your own research around melatonin. We used it at the suggestion of a respected friend for our son,and it did work. However, there are strong links between melatonin use and retardation of gonadal development. I found that when I used it, I would get strong cramps. Another trusted friend who had sleep issues and was post-menopausal started getting her periods again because of this. It's obvious to me that this is an extremely strong hormone and we stopped using it. The other concern is that this is usually a synthetic hormone that you are being sold and it's unregulated-- both of these issues are addressed here:
http://www.kidsoc.org/news/press/entry/the-dangers-of-mel...