I love Karen's gentle hack for encouraging little ones to stay in bed:
Both my kids were prone to coming out of their rooms at night as toddlers, saying "I can't sleep." A trick that worked well for us was that I would tell them to go back to bed and try to fall asleep and that I would come in in five minutes.
They each had a digital clock in their rooms so I might say, "I'll be back at 7:52 to check on you." I'd go in at the appointed time, kiss them on the forehead, thank them for staying in bed, and promise to come back in 15 minutes ("at 8:07" say).
I never missed a promised return, but usually by this second trip, they were sound asleep. It turns out that watching your digital clock change is as soothing (boring?) as counting sheep.
My son has this Oregon Scientific clock in his room that projects the digital time on the ceiling. Pretty cool.
Related: Sleep tips at Parent Hacks