Can I export my signature from Preview on one Mac and import it on another?
This was covered by Aussie Bloke's blog when Lion arrived. Here are the steps to get both the file where the signature is stored as well as the associated keychain entry to a second Mac.

On the source Mac:


Open the ~/Library/Containers/com.apple.Preview/Data/Library/Preferences folder.


In Finder, click the Go menu and hold option to show the Library folder.
Alternatively, press ⇧⌘G whilst Finder is active and enter the path above to directly navigate.

On OS X Mavericks 10.9 and earlier, copy the com.apple.Preview.signatures.plist file.
On OS X Yosemite 10.10 and later, copy the com.apple.PreviewLegacySignaturesConversion.plist file.
Launch Keychain Access
Ensure the login keychain is selected and choose the Passwords category.
On OS X Mavericks 10.9 and earlier, right-click the Preview Signature Privacy password and select Copy Password to Clipboard.  This is the password used to encrypt the signature images.
On OS X Yosemite 10.10 and later, right-click the Signature Annotation Privacy password and select Copy Password to Clipboard.
Paste it into a text editor and save the file.  You'll need to transfer this to your other computer(s).


On the destination Mac(s):


Launch Preview, open Preferences, and select the Signatures tab to ensure the default configuration files and folders have been created.
Exit Preview.  Ensure that the process is terminated.
Open the ~/Library/Containers/com.apple.Preview/Data/Library/Preferences folder.
On OS X Mavericks 10.9 and earlier, copy the com.apple.Preview.signatures.plist file to the folder, overwriting any existing file.
On OS X Yosemite 10.10 and later, copy the com.apple.PreviewLegacySignaturesConversion.plist file to the folder, overwriting any existing file.
Launch Keychain Access
On OS X Mavericks 10.9 and earlier, locate the Preview Signature Privacy password in the login keychain and double-click to edit.
On OS X Yosemite 10.10 and later, locate the Signature Annotation Privacy password in the login keychain and double-click to edit.
Click the Show password checkbox and paste the password you copied from your original machine.
Click Save Changes and you're done!


In situations where the target machine has no existing Preview Signature Privacy or Signature Annotation Privacy record in Keychain Access, you must create a new record. This is necessary when the target machine (like a Mac Pro with an older or third-party display attached) has no camera which means no Signature file can be created from within Preview. Open Keychain Access as above and do the following:


Click on the + icon to create a new entry
On OS X Mavericks 10.9 and earlier, enter Preview Signature Privacy in the Keychain Item Name field.
On OS X Yosemite 10.10 and later, enter Signature Annotation Privacy in the Keychain Item Name field.
Enter Preview in the Account Name field.
Paste the password copied from your original machine, per above procedure.
Click Add
In the newly created entry, click Access Control
Add Preview to applications allowed access
Click Save Changes and you're done!