Icing is most effective in the immediate period following an injury. The effect of icing diminishes significantly after about 48 hours.
Apply ice directly to the injury. Move the ice frequently, not allowing it to sit in one spot.
Keep the injured body part elevated above the heart while icing--this will further help reduce swelling.
Ice for 15-20 minutes, NEVER LONGER. You can do more damage, including frostbite, by icing for too long.
Allow area to warm for at lease 45 minutes before beginning the icing routine again.
Ice as frequently as you wish, so long as the area is warm to touch and has normal sensation before repeating.