You can find the answer in this
post, but I also document it here so I'll have a permanent copy.
Let's suppose you've got an error like this:
W: GPG error: http://ftp.de.debian.org stable/non-US Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D
W: You may want to run apt-get update to correct these problems
You should run the following to fix it (substitute F1D53D8C4F368D5D
with the one from your error message):
gpg --keyserver subkeys.pgp.net --recv F1D53D8C4F368D5D
gpg --export --armor F1D53D8C4F368D5D | apt-key add -
Now you can run
and will not get an error message.