Use secure-apt with unofficial repositories
If you use unofficial repositories (such as mine or testing-security), your favorite apt front-end (eg. aptitude or synaptics) complains/warns about packages checking.
To use this new functionality, and make apt stops complaining, you just have to import repository maintainer’s gpg key in your keyring and let apt know about it:
Simple steps
As root, do:
apt-key adv --keyserver hkp://the.earth.li --recv-keys <key_id>
Note that you should replace <key_id> with the public key id you want to import (I use E435C74B to sign meta-files of my repository).