My tree is over 40 feet (12 miters) tall and produces 700-800 avocados a year, I have done quite a bit of experimenting and I have learned to make my guacamole the same way as it's served when I visit Rosarito, Mexico:

1 Avocado, ripe

1/3 Onion, medium, finely chopped
1 Garlic clove, minced or crushed
1 Tomato, small, chopped into 1cm (1/2-inch) bits
1 tbs. (15 ml spoon) Cilantro, aka Coriander(to taste)

1/4 tsp. (1.25 ml spoon) Salt (to taste)

Mix thoroughly! The avocado should have a few chunks to give the mixture variations in flavor from bite to bite. Multiply recipe to make larger portions, one avocado is barely enough for one adult serving.

I have heard of people sticking sour cream in their guacamole to "stretch" the avocado, but I personally think it steals from the flavors of the avocado. I have also met some Mexicans who put lime juice (limon in Spanish) into their guacamole, because they like it sour. I have never heard of putting chilies into guacamole. (Well, actually I have, see A949340. My belief is that if it's spicy it's salsa, that's all.) Some Mexicanos also add tomatillos to their guacamole.

Tips for successful Guacamole:

Avocados

An avocado should dent just a bit when you squeeze it. If it is hard, it won't have flavor and won’t mix well. The easiest way to "peel" an avocado is to cut it in half the tall way (make a ring around the big pit in the middle) and give the two halves a twist opposite each other. Get the pit out by banging the knife blade against it and twisting the pit round in the meat. GENTLY push the knife through the meat to the skin and make neat vertical slices about 5cm (1/4-inch) apart through all the flesh, don't cut the peel! Get a spoon and scrape out the sliced meat (the slicing helps keep the skin from tearing and sticking on the meat).

Tomatoes

Roma Tomatoes are perfect for guacamole, lots of outside bits, which is what you want, and what inside there is, is firm. (Extra tip, avoid refrigerating Tomatoes and Avocados, it changes the flavour).

Final Adjustments

Taste mixture first, if it's pasty or powdery, add a pinch of salt and mix well. Repeat until you can "taste" the avocado. If you've never had an avocado, buy extras, you will screw up the first time. Also, the flavour can be subtle, especially on store-bought avocados so try different types. In my experience Haas Avocados have more flavor, but good Green Avocados can make excellent guacamole. Some people take time to like avocados, others like them right away.

Good guacamole should have a big bang of flavors, and finish clean.

A Tree of Your Own

If you want your own tree, get several unrefrigerated avocados, move somewhere warm and set them *on top* of some good, soft soil. Cover them with loose leaves to keep away the possums, who LOVE rotting avocados. Water daily, use rose or tomato fertilizer sparingly once the tree is growing.

A great site on the history of the avocado, suggested by Anhaga:
http://www.mexicofile.com/holyguacamole.htm

Some alternate entries on avocado dishes:

Guacamole A949340 by Mich

Avocado Salsa A413641 by Chance

A metric conversion table for recipes:
http://www.allrecipes.com/cb/ref/convert/conversions_met.asp