Show your son the new bed and tell him how exciting it is to be in a big boy bed!
Both of my kids had practically no problems at all. My son woke up the first night and was crying. We still had the crib set up, so we asked him if he wanted to go back to his crib and he said no, and went back to his big bed. That was it. We left his toys in his room and found him playing a couple times, but we would firmly tell him that wasn't o.k. and put him back to bed. It really wasn't that big an issue. We also had to transition him due to our daughter on the way.
Our daughter was even easier. We put her in it about a month ago, and we've had three minor incidents. The first was that she climbed out of the bed and was standing between the end of the bed, the wall and a box fan. She got scared because she didn't know how to get out of the spot. Then once, the bed got pulled away from the wall and she rolled into the gap and got stuck. We pulled her out and put her back to bed, and she was fine. The third was when she rolled out of the bed onto the floor. She was just surprised by it, and went right back to sleep.
I think how you decide to transition depends on your nightly routine. If you plan to change the bed time routine, you might want to do that before changing beds. Then again, some kids just go with the flow . . . :) You know your son. Make an educated guess on what will be easiest for him - a slow transition with an option to still stay in the crib for a little while, or completely transitioning all at once. We gave our son the option. We threw the crib out the day we put up our daughter's bed (it needed to be tossed anyway). As someone else posted, it's always a bigger deal for the parents than the child. :)
Good luck, and sweet dreams!