Today I learned that you can't copy a Mac application's plist by just copying the file. However, you can export the plist and import it on a new machine:
Step 1: Exit the app.
To make sure the file is stable.
Step 2: Export the plist data:
$ defaults export info.colloquy ~/info.colloquy.backup
To know the name of the plist (info.colloquy in this example) look in ~/Library/Preferences. Use the filename but strip off the .plist suffix. If an app has multiple plists, (I assume you need to) do each of them individually.
Step 3: Copy the backup file to the new machine
I like to either copy it to Dropbox and wait for it to sync on the other machine, or scp it to my VPS and then scp it down to the new machine.
Step 4: Import the plist data:
$ defaults import info.colloquy ~/info.colloquy.backup
Step 5: Start the app and make sure it worked.
Because we're adults and we check our work.
Personal I prefer MackUp - but you might want todo that by hand and not with a tool that takes care of some applications.
https://github.com/lra/mackup