How I clean up a Debian system

Debian systems can accumulate a lot of cruft. Fortunately, it is quite easy to clean up your installed packages. Most people would probably be okay with this:

aptitude markauto '~i!~M!~nbuild-essential(~E|~prequired|~sdevel|~sinterpreters|~slibdevel|~slibs|~soldlibs|~sperl|~spython)'

You can then see which non-essential packages are installed with

aptitude search '~i!~M!?essential'