Photo Credit: Isaak D. Edmonds

What you need:
.ice
.zip lock bags
.salt
.ice cream toppings
.syrup (optional)
.vanilla extract
The Science:
"Ice has to absorb energy in order to melt, changing the phase of water from a solid to a liquid. When you use ice to cool the ingredients for ice cream, the energy is absorbed from the ingredients and from the outside environment (like your hands, if you are holding the baggie of ice!). When you add salt to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt. This makes the ice colder than it was before, which is how your ice cream freezes"


http://chemistry.about.com/cs/howtos/a/aa020404a.htm