I have a book that I have read parts of at various times: Healthy Sleep Habits, Happy Child by Marc Weissbluth M.D. It is one BIG reason why my son sleeps pretty well now and, if I have anything to say about it, far into the future.
When the boy was several months old, getting him to sleep at night was a nightmare. We had first trained him early on (like, 6-8 weeks of age) and experienced early success. Sleeping 12 hours a night at 4 months old was due to part training, part natural tendency (I don't think I can take all the credit since every child is different). We used the book and got him back on track.
However, recently his bedtime ritual has included several drinks of water, more stories, longer back rubs, blah blah blah. I'd had enough. I picked up that book and searched for our situation: reluctance to go to bed. Whatever the reason, whatever the age, their recommended method for retraining is the same: extinction. As in, quit reinforcing that behavior and leave him alone when it's time to go to bed. Does he cry? That's okay. Give him time to settle himself down -- he's apparently forgotten and needs to be reminded.
My poor, exhausted husband emerged from the boy's room last night, and I pointed to this extinction method. Sometimes us parents need to be retrained too! So we left him alone. Did he cry? Yep. We ignored him. Did he stick his fingers under the door, randomly whine, and say "Hi Mommy...?" Yep. Ignored him again.
Within 10 minutes he gave up and went to sleep on his own. Training and a lock on the outside of the door work wonders.