Avoid buying overripe avocados because they might be bruised under the skin. This makes for an unpleasant, browned color and flavor, which isn't optimal for guacamole.
When you make guacamole with tomatoes, it's important to remove excess moisture from the tomatoes to prevent the dip from becoming too thin.
When making guacamole ahead, you'll want to prevent oxidation. Simply place guac in an airtight container, pour a thin layer of water or lime juice over the surface, cover, and refrigerate up to 24 hours. Drain and serve.