Well, first let's talk about why avocados turn brown. Like apples or potatoes, they oxidize when exposed to air. Once you cut into an avocado, you'll never be able to completely stop the oxidation process, but you can dramatically slow it with a few quick tips and tricks:
1. Cut the avocado with a ceramic or plastic knife. Metal actually accelerates the oxidation process.
2. Try lemon or lime juice. Citric acid is a powerful antioxidant; rub a little juice around the exposed flesh and you'll significantly delay the browning effect.
3. No lemon or lime? Use oil. Oil is another great buffer to oxygen. Use in place of lemon or lime juice when you don't have any handy.
4. Store as air-tight as possible. Again, avocados turn brown due to oxidation and exposure to air, so storing in an air-tight container only makes sense.
5. Water. Huh? That's right! This one works exceptionally well for guacamole. Place your leftover guac in a plastic container and press down to remove any air pockets. Add a half inch of water on top and seal with an air-tight lid. The water creates a barrier between the avocado and the air, keeping your guacamole fresh and 100% green for 24 hours or more! When ready to eat some more, just drain the excess water and enjoy. Works like a charm!