Potato juice: Wash the sunburned area thoroughly and dab it dry. Don't rub it with a towelette as that might increase irritation. Grate potato without peeling it, squeeze the juice out of the grated pulp, and apply it on the affected area.
Cold milk compress: Take a washcloth and dip it in really cold milk. Take it out, drain the excess liquid and keep it on the sunburned area. Keep it till the cloth reaches room temperature and then repeat the process. You will feel better immediately.
Baking soda: If the area has a burning / stinging sensation, the best option is to apply baking soda. Depending on the area of the burn, mix some baking soda with water and make a paste. Apply the paste on the burned area and keep it until the stinging reduces.
Tea and mint leaves: Boil some water and add tea leaves to it. After the water changes colour, add mint leaves and let it boil. After two minutes, strain the liquid and keep it in the fridge to chill. When the liquid is really cold, pour it over the sunburned area or dip a cloth into the potion and apply it on the area. Read more on idiva.