We have a 3.5 yr DD that we used to stay with till she went to sleep. We also have a 1 yr DS and, theoretically, they room share.
When DS came along, DD was in the process of dropping her day sleep (still not entirely dropped) so a couple of months ago we grabbed the opportunity to discuss DD going to sleep by herself with her. I think at 3, she was old enough to understand what we were saying and go cold turkey. She did it the first couple of nights no problems (we gave her heaps of praise for doing so, got nanna to comment on it etc etc), but then she started to jack up a bit about it. We were pretty firm about her going to sleep by herself, and only very occasionally has DH sat with her for five minutes only to get her to settle down a bit.
It all seems to run quite smoothly if she hasn't slept during the day, if she has slept we put her back to bed 100 times.
The other catch is that she mainly goes to sleep in our bed, and we transfer her later or she stays in that bed (I sleep in her room with DS cause he still feeds at night). To be honest, it's a work in progress, mainly because we play musical beds here. Not sure what we will do with DS later (we rock him to sleep) as they go to bed at the same time and I can't imagine that running smoothly- we will probably stick with the going to sleep in different rooms. I have bought a star projector and am thinking we might give it a try to help DS self settle and it may help us getting them to sleep together....