For Xcode5 it is very simple.
In Xcode Preferences select Devices. Click on the gear icon at the bottom and select "Export Accounts". Select a save file name and location, add a password (really do add a good password because this file will have your private developer key in it).
On the new Mac go to Accounts and the gear and select "Import Accounts".
There is more complete information in the Apple Documentation for Exporting and Importing your developer credentials.