Consistency is the key! You have to make sure each and every time the child gets out of bed you immediately take them back to their bed without disciplining or playing with them. Be consistant and you will be successful.

It's okay if you want her to sleep with you occasionally.

My daughter is also 6yo. She lives with her mother in another state and mostly sleeps on her own, but with me she likes me there. Probably because it's not her usual bed nor home to sleep in and partly for the comfort.
My ex-wife's son used to come into bed at early hours of the morning. I'm a light sleeper so when I heard him get out of bed, I'd wake up and go into his room until he fell asleep again then back to my bed. Usually this was once a night, so it wasn't so bad. In the end, though, I decided he needed to learn to sleep on his own.

One morning I walked out and shut the door after telling him he would need to sleep on his own from now on. He cried for about half an hour until he was so tired he fell asleep. It nearly broke our hearts, but had to be done. From that night onwards he stayed in his bed every night. He learned that it was safe to be in his bed by himself and he could get himself off to sleep with no trouble so he didn't really need us any more.

It could be harder for my daughter as she is moved from home to home and sleeps in different beds, plus I'm a softie when it comes to her (my ex-wife's son was not mine. He was from a previous relationship).

If it doesn't worry you to have her there, then there's probably no problem. If you prefer her to sleep in her own bed you may have to try the above, but you will both probably be crying on either side of the door. It's up to you.

You made your job far harder by waiting so long, by age 6 the behavior is pretty ingrained. What you do have on your side is the age of understanding. By age 6 she can understand when you reason with her and your reasoning. I would explain why you want her sleeping in her own bed and then reward her sleeping in her own bed. Example: If you sleep in your own bed the entire week, we'll buy you a whole new set of awesome sheets! this both makes her bed cooler, and gives her more of a reason on top of your logic to want to sleep in her own bed.

Keep in mind though she'll probably crawl into your bed when she's had a bad dream or doesn't feel well anyway, even kids that didn't co-sleep will. That shouldn't be punished she knows you're there for her. I actually wouldn't recommend disciplining the getting out of her own bed at all, simply reward not getting out.

I had to break a kid of that habit.

It's gonna be different for every kid and for every situation, so there's no way anyone can say how likely this is to work... all I know is that it worked for me.

I told the kid that now that he was big enough to be in school, that part of the school rule was that he had to sleep in his own *blankets*, so he had to bring in a mat and blankets with him, and sleep on the floor next to my bed.

If the kid crawled in, I'd lift him out and put him back on the mat and tuck him into his own blankets. I had him think that the issue was the blanets.

It took about ten days, but it worked.

Tips for Helping Children Sleep in their Own Beds

Many families throughout history have chosen a "family bed." In fact, in most cultures around the world today, a "family bed" is the norm. If that is a family's choice, it can work very well. However, having a "family bed" is not for everyone. It also works well to teach children to sleep in their own beds. Here are some tips on how to help children sleep in their own beds:
It does not work well to tell children to sleep in their bed and then relent when they act up. This only teaches them that their persistence will be rewarded with a trip to your bed.
The best approach is to discover the underlying cause or causes for your child's behavior. At some level, your child knows why she doesn't want to sleep in her own bed, even if she isn't able to articulate what she is feeling. If you ask her outright what she is feeling, you may not get any valuable information.
A great way of discovering what your child is feeling is to play with her using dolls or action figures to represent members of a family. Have the characters act out several typical family situations: mealtime, going to the park, driving in the car, etc. Enact several of these non-threatening situations, and let your child put words into the figures' mouths. When you get to bedtime, if your child is hesitant to talk, you can try speaking for the characters. If your child has gotten into the play, she will correct you if you give the characters motivations that are inaccurate from her perspective.
Another approach is to encourage your child to color or paint while she tells you about what she is creating. Be sure to allow her lots of time to open up and don't react negatively if she says something you don't want to hear.
The things you are most likely to find are: your child has night time fears, i.e. she is afraid of the dark, being alone, closet monsters, etc.; she is jealous of one parent or a sibling; she is afraid of losing your affection if she "grows up"; or some variation of one or more of these.
If your child is suffering from night time fears, give her tools that empower her to overcome her fears:
o Give her a flashlight to play with (especially during the day in a darkened room) to help overcome fear of the dark.
o Give her a spray bottle filled with "monster spray" so she can shoot the monsters if they come out.
o Record a tape of her favorite stories and songs that she can turn on whenever she is feeling alone or afraid (it is best if the recording is of your voice).
o Give her a stuffed animal as big as she is to sleep with.
o Ask her for suggestions.

If the primary reason your child wants to sleep with you is night time fears, you should be able to switch her into her own bed as soon as she has the tools to cope with her fears.
If you discover that your child is jealous of one parent or a sibling, evaluate the situation and determine if she has reason to be jealous. If she has a younger sibling who is getting most of your attention during the day, she may feel the only time she "gets you" is at night. The best way you can help overcome her jealously is to pay special attention to her when she is not asking for it.
If your child reveals that she is afraid that she will lose your affection when she grows up, take stock of what you are communicating to her regarding growing up. You may inadvertently be sending her the message that you want her to stay a baby. If this is the case, consider how you can change her feelings by the way you communicate with her.
You will need to take some time for your child’s feelings to change before you can move her into her own bed. When you do, you may need to make the change in several stages. The first few nights, she might sleep on the floor beside your bed. The next move might be right outside your door, then into her own bed. The large stuffed animal or the tape recording of your voice may help ease the transition.

I hope this helps: http://www.youtube.com/watch?v=R4FWaa7_uak