To easily peel a tomato, first cut an "X" in the bottom of the tomato (opposite the stem end), and remove any remaining stem.  Bring a large pot of water to a boil, and have a large bowl of ice water ready.  Place 3 or 4 tomatoes in the boiling water, and boil them for about 30 seconds, or until the skin at the "X" begins to pull away from the tomato, if sooner.  Plunge the tomatoes in the ice water.  Once they have cooled, remove from the water and peel off the skin.

If your tomatoes are underripe, the skin will not come off as easily.  If you are going to be cooking the peeled tomatoes, you can leave them in the boiling water longer, until the skin splits or begins to come away from the "X". If you will use the tomatoes raw, it is better to peel them using a serrated vegetable peeler.  In fact, the latter is our preferred method if the recipe only calls for a few tomatoes.

If you have a gas stove, you can also use it to peel a tomato.  Remove any remaining stem and spear the stem end with a fork.  Turn on a burner over medium-high heat, and hold the tomato about one inch over the flame, slowly turning the tomato until the skin splits.  Put aside to cool, and then peel.

Avocados (and many other fruits) turn brown when exposed to air due to the action of an enzyme. To inhibit the browning, you must either minimize the exposure to air or slow the action of the enzyme. To do the former, cover the avocado with plastic wrap, making sure to press the plastic tightly against the surface of the avocado, or spray the avocado lightly with oil or cooking spray. Citrus affects the enzyme responsible for browning, which is why most guacamole recipes include lime juice, so rubbing extra avocado with lemon or lime juice will also work. Apparently, keeping the pit in guacamole only works because it keeps air away from the part of the guacamole touching the pit, and only keeps that part of it from browning.