we moved our son at 17 months to a big boy bed...he was happy in his crib, but we were due in a few months so i planned to move him early enough....
anyway, the bed was in his room for 2 weeks....then we read bedtime stories on, talked about big boys sleeping in beds, and then back to crib for bed. one night he wanted to be in the bed...so we tried. he got up, so he went back to crib. after a few nights he learned not to get so upset so he could stay in the bed. so then we said no more crib. he agreed. so when he cried that night i did not go to him right away. he slept on the floor for a few nights, then figured out the bed was more comfy.
i admit.....hearing him play in his room after i left was hard.....my dh wouldn't let me go up there....said, its his room, let him explore. sure enough in 10 min he was back in bed asleep. never had any more issues.
but we had a one strike rule that first week....he went back to crib if he got out of bed and cried....then when it was no crib i just did not go to him right away....and he fell asleep everytime on his own......but we hyped up the bed thing before trying it out.....naps were harder b/c he wanted to play more.....but soon he got it and stayed in bed.
just be consistent and make clear the rules of the bed.....but don't be afraid to let the child play some in the room after you leave...its their room and they need to be able to explore....they think they are getting away with something ;-)