If you get an error like this:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY STRING W: There is no public key available for the following key IDs: STRING
Where the STRING is you ID
The solution:
gpg --keyserver pgpkeys.mit.edu --recv-key STRING gpg -a --export STRING | apt-key add -
Note: you can use keyserver.ubuntu.com instead pgpkeys.mit.edu