Most commercial places do add the vitamin c, as mentioned above. Supposedly, it also helps to keep the pits and put them in the container when you save it. Then definitely the plastic wrap on the surface of the guac. I've got about sixteen different ways I make guacamole. I started out with the recipe from Rick Bayless's cookbook... But tweaked it. I started adding a bunch of garlic to the tomatoes, onion, jalapeno, cilantro and lime juice. Most recently I've been super lazy and buying salsa verde and just mixing it with the avocado. It's actually really tasty. You get that great acidity of the tomatillos. Depending on the salsa, I'll add a little lime juice and salt. Always salt. This way, also, I just make one avocado and it usually ends up being dinner.