Mash avocado with a fork or potato masher in a bowl until the consistency you prefer. Add lime juice and salt and mix well with fork. Stir in tomatoes, onion, and chopped cilantro. If using cumin, add at this point.
Pour into a serving bowl and serve.
To help keep guacamole fresh, return pit of the avocado to the prepared guacamole, cover with plastic wrap, making sure to press the plastic wrap onto the guacamole to keep as much air as possible away from the guacamole. Refrigerate until ready to serve.