Place all of the sherbet in the bottom of a large punch bowl.  Pour the Sprite and Sierra Mist over the sherbet in equal amounts until the bowl is full. Ladle the liquid over the sherbet continuously for a few minutes to mix. Serve. As the punch is consumed, keep adding the Sprite and Sierra Mist to the bowl until the sherbet is gone.

Note:
What makes this punch so great is you can make it what ever color you need by selecting various flavors of sherbet.  If you want an almost clear punch that doesn't stain, use pineapple sherbet.