S.D.
This sounds silly, but we got an egg timer and put next to my son's bed. We'd set it for 15 minutes, read him his books, and when the timer went off, it meant it was time to go to sleep. The fact that WE weren't telling him arbitrarily that he had to go to sleep seemed to do the trick.
Seriously, he had no concept of how long the timer had been ticking, so we were just arbitrarily choosing how long until he had to settle himself, but he didn't catch on. It just worked. It worked so well, we started using it to get him to clean up toys and move on to other activities.