Foaming at the mouth and falling over backwards
Foaming at the mouth and falling over backwards (or, in internet slang, FATMAFOB) is an art form which has been compared many times to the Stoner Tea Ceremony. The exact procedure for using it to greatest effect is a secret that is rumored to be held by Pee Wee Herman. However, it's three basic steps can be discussed here.
The Stick Pose
Stand straight up as best you can, such that you look like Al Gore. WITH the stick up his ass. If you are doing it properly, your thighs should feel just a little tense. This is critical to the third step, so remember to do it, or people will laugh at you when you simply foam at the mouth and DON'T fall over backwards.
Foaming at the mouth
Close your mouth and remove all expression from your face. Then open it just a tiny bit and let spit almost escape your mouth, such that it forms a filmy, bubbly, clearly visible mass at the mouth. (If you desire a more dramatic solution and are willing to sell your soul to Satan, put a large amount of shaving cream in your mouth and just let it out for an excellent foam.)
Falling over backwards
Relax your thighs, and you'll naturally be pulled backwards. Once you're solidly falling, lean forward to make the landing look more dramatic while also making it less painful. Don't let one leg intersperse itself to prevent your fall, or you will look horribly lame and will have your pants removed by the Board For The Correct Implementation Of Foaming At The Mouth And Falling Over Backwards and/or Masters of the Internet.