My son was 2 when he moved from the crib to a twin bed. We approched it as just a normal step in growing up. My son loves story time, so we got a few books about getting a new big boy bed.
Sesame Street Big Enough for a Bed: This was great about explaining the reason and process of getting a new big boy bed. Only complaint- Elmo's Mom's head is cut off in some of the illustrations. It sort of freaked my son out a bit to see those pages.
Your Own Big Bed by Rita M. Bergstein: This was the best of the books we got. It uses different baby animals growing up and compares it to a little boy in the story. Getting a new bigger bed is the last stage of growing in the book.
My Own Big Bed by Anna Grossnickle Hines: This is written from the viewpoint of the little girl getting the new bed. It shows potential scary points that the girl has about getting a new bed. It also shows how she thinks of ways to overcome those problems. I got this book just in case, my son felt scared about getting a new bed. I decided not to read this book to him unless he showed that he was scared. We ended up not reading it until long after he had been in his new bed for about a year. But it had great ideas of how to approach those scary feelings, so we bought the book just in case.
We read the Elmo book and "Your Own Big Bed" book for about a month before making the switch. During that month, we took our son shopping for his new bed and sheets to go with it. We also allowed him to choose one new stuffed animal to be is new friend in his new big bed. He chose his bed, sheets, and friend on his own. We bought two sets of sheets in colors that he liked. (Although, we did say no to the pink sheets, just in case he later complained as to why his sheets were pink. He chose yellow and green instead. He is five and still likes the color pink. When he got his own trashcan for his room, we let him get a pink one.)
On the day the bed arrived, our son helped us clean his room completely to make a space for his new bed. He said good-bye to his crib. Because of the layout of his room, we let him change where he had the pictures on his wall. We added some new pictures, and kept some of the old ones. He decided which sheets and blankets would go on the bed first.
We explained that we expected him to go to sleep in his new bed just the way he did in his crib. We told him that we would come get him up in the morning just like we always had. We told him that we could still see him through the baby monitor and hear when he needed us just like we always had. We kept the bedtime routine the same, except that at story time we now sat on his new bed because the rocking chair wouldn't fit in his room with his new bed there now. His new stuffed animal friend replaced his older lovey, although his old friend sat on the backboard of his bed watching over him, while he slept with his new friend. We did not show nervousness to him at all when putting him to bed after he got his new bed. It was just the attitude of "this is normal, no problems." He never got up once to come and get us. In fact, it took about a year before he busted into our room one Saturday morning at 5 am demanding breakfast. That was when we bought him an alarm clock and explained that we wanted him to try to sleep until the alarm went off. We would let him read or play in his room at that time if he didn't feel sleepy, but he had to wait until we got up to eat breakfast. He was good and tried to sleep for awhile, but sometimes we found him looking at his books or playing when we got up. We had doors to the living room, bathroom, and study locked or blocked off with a gate. He had access to the toilet and our room only. (We live in Japan. Toilets are not in the bathrooms in Japan.)
Make the process of getting the bed exciting for him, but present it as a normal step. Keep your betime routine the same as always, and keep a "This is normal. We expect you to stay in bed. We know you can do that." attitude. The books greatly helped us make that transition smoothly. They were worth every dime spent.
Good luck.
