In my experience, there are 2 distinct ways to lunge. I've always divided it into classical lunging and natural horsemanship (NH) lunging. Mercedes is outlining classical lunging, which I agree is every bit the art that classical riding is. Classical lunging is far more useful than NH lunging if you are lunging to give the horse real, correct exercise as an alternative (using the word alternative very loosely here) to a mounted schooling session.
NH lunging I find is most effective for very green or very disrespectful horses. The point here is not to build muscle or develop a good frame. What NH lunging contributes is respect and rapport. Not that classical lunging doesn't, but classical contributes to the physical development of the horse in addition to the mental and emotional. I personally have more experience with NH lunging, having handled mostly young or uneducated horses, unready mentally and emotionally to respond well to classical lunging. My personal horse is now very respectful, moving out of the grass green umbrella, and I thank Mercedes for the direction to the Klimke book as I begin my foray into more classical lunging.
So, to the original question: What is the proper way to lunge? It depends heavily on what "style" you want to lunge. NH lunging has so many brands and variations (almost all of which will get the job done if applied with common sense, feel, timing, and experienced help on occasion), type almost any name brand trainer into a search engine and you'll find a plethora of info on their slant to lunging. Several posters here have given excellent overviews of the basics. If that's what you're interested in, I recommend Clinton Anderson's Lunging for Respect stages 1 and 2; Pat Parelli's Circle Game; or almost any roundpenning series. NH basically treats lunging like roundpenning with a line. Not a bad thing, just different and used for a different purpose, to reach a different goal in horsemanship. It isn't "easy" per se, but is much more learnable from a DVD or study kit than classical lunging.
If you're looking for more classical lunging techniques, your best bet would be to look into a trainer and have a few lunging lessons, both lunging an experienced classically lunged horse and with your own horse. The adjustment of things like surcingles and side reins can be tricky enough to figure out alone, without touching on learning how to cue precisely, etc. I advise some research into classical dressage in general; that topic almost certainly will guide you into classical lunging. As I'm learning about classical dressage myself, I can't really give any specific advice about it other than to continue to research.