I read your post on helping a toddler move gradually into her own bed. I tried this method, but here's my dilemma: my daughter started sleeping in her room after her bedtime routine but, at about midnight, she's in our bed again. She has never once slept through the night in her bed despite me hinting to her that it is OK for her to put herself back to sleep since mom and dad are next door.
Do you have any other advice on how to help her sleep through the whole night in her room?
Sounds like your daughter makes it through her deepest sleep (which is at the beginning of the night) and then is in the habit of joining you. If you want her to sleep independently for the full night, here's my advice:
It's good that you first got her in the habit of falling asleep in her own bed, after a nice bedtime routine, without you there. (If you were there while she fell asleep, she'd be much more likely to want that again when she moved out of her deep sleep).
Sometimes just starting the night independently is enough, but it sounds like your daughter needs a Plan B. You've already got a bedtime routine, which ends with her feeling confident in her own bed. Make sure the environment is dark and cool, and that nothing changes in her bedroom between when she falls asleep and when she wakes at midnight. Put her to bed early (between 7 and 8 p.m. works well for most little ones).
After that, if you don't want her sleeping with you, I'd recommend walking her back to her bed each time she comes to yours. In our sleep consultations, we help parents figure out the most gentle and clear way of doing this. But the goal is for it to feel very monotonous and non-engaging to the child. No frustration or negotiating—the tone is very important—simply return her to her bed.