Well if you want to try and break into your own box
a) search for a known exploit for the
software you are running
b) get the source
code to the programs you are running and look for
mistakes that could lead to a buffer overflow or other
This article will help you craft your own exploits:
Other things to do are subscribe to security mailing lists
Read stuff like this:
If you aren't following the advice there see why you didn't
and see if you can do something you shouldn't be able to.
If you can, see what that lets you do. Play around.
If you learn to think like an
Just do everyone a favor, if you do find an exploitable flaw
send a patch to the project.