WARNING: This could quite possibly kill you or others. You have been warned!.
The most suitable fuel for fire breathing is kerosene.
Other fuels such as meths, petrol, paraffin can be too flammable at times and taste disgusting also. (although kerosene itself is no bottle of Moet & Chandon)
The best way to learn fire breathing is to practice expelling liquid from the mouth in as fine a mist as possible, ensuring the liquid is expelled far away from the body also.
A good way to practice before you go setting anything (or anyone) on fire is to practice the spray with water.
Take a sip of water from a glass, make a small reservoir with your tongue and lower teeth and hold the liquid there. Inhale deeply through the nose, then as hard and as quickly as possible, blow the liquid from your mouth to form a fine mist. This is the basic action of fire breathing.
When you have perfected the spray, you are ready to try it with the real thing.
Find a non-flammable stick like object, about 10 inches long, anything will do so long as it won't melt of or go on fire. Wrap some cotton wool around the top of the stick.
Take a few swigs of the kerosene (without swallowing of course!), to get used to the taste, remembering the practice with the water.
Take a small swig of kerosene to begin with, dab the stick with the cotton wool in a little kerosene, tilt your head back, so as not to burn the hairs on your nose, wipe any excess fluids from your face and neck. Stay calm, then blow out as hard as goddamn possible, expelling the mist on to the flame and igniting it.
That's it, you should now be able to breathe fire. It does take a bit of practice to master, so be paitent, and most importantly, careful.
This is the key.
Safety Tips:
lj says: in fire breathing, you might want to mention that shaving, and tying back hair is a good idea. Having burnt stubble just looks stupid :) (and I've seen a beard act as a wick before, with unpleasant consequences).