I was browsing the Internet this morning and came across this how to from Kevin Tisue on About.com

A torn mountain bike tire casing or sidewall on the trail can spell disaster for the rest of your ride. When you tear your mountain bike tire casing on a rock or other sharp object your tube usually blows out through the tear as well leaving you with a blown tube and a hole in the side of your casing that will blow through the rest of your patches and tubes as well if you don’t know what to do. Here’s a handy trick that will get you moving again.

1. Remove the wheel with the torn mountain bike tire casing.
2. Use your fingers or tire levers to remove bead from the rim on one side of the tire.
3. Leaving the tire on the rim, pull out and patch or replace the blown tube.
4. Fill tube with just enough air for it to take shape.
5. Replace valve through the valve hole of the rim and tuck the tube back into the tire.
6. Take an energy bar wrapper, energy gel wrapper, dollar bill, or other tough material, fold it a few times for extra strength but make sure to leave it much larger than the tear in the mountain bike tire casing. Tuck your folded reinforcement in between the tire and tube so the tear is at the center and you can see your reinforcement through the tear.
7. Replace the free tire bead back into its position along the rim using tire levers if necessary.
8. Pump the tire up about half way and check for proper bead placement around the rim and make sure your reinforcement is still in place and holding.
9. Finish pumping the tire up to a rideable pressure. If your tear in the mountain bike tire casing is small you may be able to pump it to your normal riding pressure. If the tear on your mountain bike tire casing is bigger you will have to limp home on lower pressure and make sure you don’t pinch flat on the way. You will be able to tell how much pressure your reinforced tear can hold by how much your tire stretches and distorts around the tear.
10. Replace the wheel.
11. Ride on.

Read more how to’s from Kevin Tisue.