Place a handful of arnica flowers inside of a thin cotton cloth such as an old T-shirt or a milk cloth. Fold the cloth shut or rubber-band it shut. Bring 2 cups of water to a boil in a small sauce pan. Remove from heat. Lay arnica poultice in the water to steep for about 5 minutes. Lift out of the water. Drain off excess water and cool until it is a pleasant temperature for laying against the bare skin. Lay poultice on bruises, sprains, soft-tissue injuries.
Make a poultice the same way using Comfrey, for bone bruises and breaks. Even for animals!
Can be repeated as often as you want, as long as no rash or discomfort occurs.