You can't, but you can slow it down. Use Hass Avocados and always add lime to guacamole. If not serving immediately, cover the guacamole with plastic wrap, pressing the wrap so it's directly on the surface. Keep it cold as warm air is guacamole's enemy. Limeless, uncovered guacamole at room temperature will turn dark pretty quickly. I've never found the avocado pit trick to work at all, aside from annoying guests.