Start by making sure you have the defendant's proper address (you want to ensure that your judgment is not going to be set aside later on because you served your small claims complaint on the wrong address).
Hire a process server to do a "skip trace" on your defendant - they can both confirm the address, and if necessary serve the SC-133, and file a proof of service with the court.
Assuming you have the right address, and there is no problem with your judgment, you can then take steps to enforce your judgment:
If somehow you did have the wrong address, you will want to confirm that you properly served the defendant with the complaint (check very carefully to ensure that the defendant was properly served: http://www.courts.ca.gov/9742.htm
Again, assuming that the defendant was properly served, but they simply moved - go ahead and start enforcing your judgment.
If you have the wrong address, and you did not properly serve the complaint at the start of your suit, you are going to need to start all over. (If the defendant appeared in court and defended the action, you don't need to worry about this - they appeared and service of process will not be an issue!!).