Note: This version is correct for MacOS 10.8 Mountain Lion. For MacOS 10.12 Sierra, check out Modifying Email Signatures in 2017.
I blogged some time ago about the process of creating my email signature from scratch. But what about when you just need to duplicate and modify a signature in Apple Mail? It's a lot simpler:
1. Go to Mail>Preferences and click the Signatures tab.
2. At the bottom of the second column, click "+" to create a new signature. Don't do anything to it right now.
3. Quit Mail.
4. In the Finder, navigate to (your username)>Library>Mail>V2>MailData>Signatures. The signature names (the ones with a .webarchive extension) are complete gobbledygook, but don't worry. You'll see one with a Date Modified of just a moment ago. Click on that one to select the file name.
5. Copy the file name.
6. Delete the file.
7. Now, click one of the other .webarchive signature files. Hit the space bar. This will bring up the Preview window. Scroll until you find the one you want to modify.
8. Duplicate that .webarchive signature file.
9. Reopen Mail and go back to Mail>Preferences>Signatures. You'll find the duplicated signature in the list.
10. Change its name to whatever you want.
11. Make the changes in the text of the signature.
12. Copy it into all of your email accounts where you'll be using it by simply dragging it onto those accounts in the left column.
13. Close Preferences... you're done!