3 to 4 ripe avocados, depending on size
3 to 4 teaspoons lemon juice, to taste (lime can be substituted)
2 small tomatoes, chopped finely
1/4 cup finely chopped onion
1/4 cup finely chopped cilantro, optional
1 teaspoon garlic salt 
Salt and pepper to taste

 The key to great guacamole is a good, ripe avocado.  Use Mexican avocados; California avocados have a different texture.  To test for ripeness, hold the avocado in the palm of your hand and squeeze very gently - it should feel yielding but not too squishy, with no sunken spots.  

Cut each avocado in half and remove the pit (using a teaspoon, scoop around the pit and pop it out).  Using a spoon, scoop the flesh out of the peel into a large bowl.  Sprinkle the avocado flesh with lemon juice.  Mash with a fork or potato masher until creamy.

Gently but thoroughly mix in tomatoes, onion, cilantro (if using), garlic and salt and pepper to taste. (You can also add a teaspoon or two of your favorite salsa.) Serve immediately.