Wow! It seems we're a wee bit behind the curve. My son is 1 month shy of his 3rd birthday and we are not yet officially moved into the big boy bed.
My son has a very difficult time with transitions so for us, this has been a very gradual process with each step determined by him.
We setup his "big boy bed" in the same room as his crib and let him play in/out/around the frame. Then we brought in the mattress. Of course, he loved exploring it but was never ready to sleep in it. We setup the rails and then special sheets (kid-themed stuff). All along, he showed an interest but always wanted to sleep in his crib. To help get him comfortable w/ his bed, we would cuddle and read stories. I'm sure the pace was somewhat determined by the bed we are moving into. We didn't start with a toddler bed - I didn't want to do this twice. We are moving from the crib to a captains bed.
Tonight, we read stories and we both fell asleep (which we NEVER do). When I got up, I told him if he wanted to sleep in this bed he needed move closer to the pillows (he was near teh end). Otherwise, I would help him into his crib. He moved towards the pillows, I tucked him in, and that's the last I've heard from him tonight :) It's our first official night in the big-boy bed.
More important than anything is to follow your childs queues. Some are much more flexible to change than others. If you do force something to happen when they are not quite ready, you may start to experience problems or regressions in other areas where they feel they can still control the outcome.