Howto Fix Error "...no public key available..." on Debian Wheezy

If when running an apt-get update on your Debian Wheezy you are getting an error similar to this one:

W: There is no public key available for the following key IDs:
7638D0442B90D010

Here is what you should do:
apt-get install debian-keyring debian-archive-keyring
apt-get update