Slice and pit the avocado. Sprinkle a bit of lemon juice over the half you won't use and put it in a plastic bag in the refrigerator so you can use it later (within a day). Mash the half for the mask and add mayonnaise. Stir the two ingredients together in a bowl until they make a smooth combination.
Add the lemon juice and stir. If your skin is normal or oily, add more lemon juice for added astringency.
Spread evenly on clean, damp face and leave for about 10-15 minutes. Rinse well with warm water. Tone and moisturize as usual. Make a sandwich by spreading the leftover mask onto two slices of bread. Add sliced cucumbers, onion and tomato, srpouts -- cheese if you wish -- and enjoy!