As someone that worked in restaurants I can tell you that avocados turn brown in restaurants as fast as they do at home! In the kitchen, cooks always joke around that 2 minutes after cutting into an avocado, the avocado is dead! Obviously that is just a silly joke. The best thing to do is squeeze fresh lime or lemon juice over the cut avocado and keep it covered with plastic wrap in the refrigerator. If you still have the half with the pit, store that one and use the one without the pit first. Not removing the pit helps in keeping them looking fresh just a bit longer. Unfortunately, there is no other magic secret that has ever worked consistently.