Guacamole oxidizes—turns an unappetizing brown—quickly without proper precautions. The acid in the lime juice or lemon juice called for in most guacamole recipes
helps delay oxidation, but not enough so make the guacamole ahead of time without it forming a brown top layer. Many people recommend keeping the avocado pit in the guacamole until it's ready to serve, which also helps a bit.
A fool-proof method to keep guacamole green, even when it's made well in advance, however, is to simply cover it with plastic wrap, making sure the plastic wrap directly and completely covers the surface of the guacamole. Store in the fridge up to overnight with nary a brown spot in sight.
Looking for a great guacamole recipe? Try one of these tasty guacamole recipes.