I have a 3 year old and a 4 yr old(though she will be 5 next month)
Bedtime is 8, 8:30-ish.
I try to start around 7:45 with books, chocolate milk, brushing the teeth, PJs.
I usually let them play quietly in their rooms for 15 minutes or so, which gets taken away the next night if they don't go to bed when it's time to go.
My daughter(older) gets extra playtime(in bed-reading playing with her leappad etc) while we sing to our son, and then she gets songs and then it's lights out.
*Usually* they do okay, come out once or twice to get a drink of water or use the bathroom, but they go right back in most of the time, so I don't mind.
My rule is, lights out by 8:30, you don't have to go to sleep, but you have to stay in bed.
We used to have quite the fight, but consistantce has been key for us!