Or, How Not to Get Killed
Surf the net, and you'll find varying information on what constitutes safety underground. Some people simply leave it at "use your common sense" and some people give long lists of things that will kill you. When it comes to equipment, the situation is the same. From long checklists to "grab a light and go," you'll see all sorts of recommendations.
What this comes down to, of course, is that people have their own styles. So, the following is intended to serve as a guide, but feel free to modify to suit your style. Things which are important are noted as such. Happy exploring!
Links to additional info:
General Safety information from OSHA.