The absolute First Rule of a Gunfight, in Mark Moritz' brilliantly enunciated aphorism, is "Have a gun!" The rest will supplement that first rule....
Have a gun.
Preferably, have at least two guns.
Bring all of your friends who have guns.
Anything worth shooting is worth shooting twice. Ammo is cheap. Life is expensive.
Funny but oh so true:
http://www.thegunzone.com/gunfighting.html