You need to stop the oxidation oand you do that by eliminating exposure to the air. If you're not serving your guacamole right away, pack it tightly into a container and top with about 1/2 inch of lukewarm water; store it in the refrigerator. This stops the air from oxidizing the guacamole. When you're ready to serve, just pour off the water and stir in the extra moisture that remains.