The first source of information is the official Apt Howto. You probably have it installed on your system someplace and it should be bookmarked (along with the Aptitude Users Manual).
Read it here
I prefer Aptitude as the program installer, and examples of Apt commands will reflect that. If you are capable of running a mixed system, you are capable of translating that to apt-get, or even Synaptic or Adept. If you are determined to run a mixed-Stable system, you should be able to make the necessary translations for that, as well.