How many times have you felt foolish for not having backed up that little configuration file you've just managed to break? How many times have you lost all of your program's preferences after installing misbehaving software or even after accidentally deleting some configuration file? How many times has your computer stopped booting after that bleeding edge kernel testing you were doing? I have, many times... actually more than I would like to admit :-)
There are some people who recompile their kernel or tweak their configuration files every day of the week, every week of the month, every month of the year. You might not be one of them but, believe me, some day you will want or need to do that; so let's assume that these are not uncommon scenarios in every day GNU/Linux life. All of them can be managed without any hassle at all if you use a little common sense and follow some practices and guidelines we will introduce you to. These will help you when those times come.
So, on to the basic things you need to have ready...