Cut a piece of plastic screening, big enough to cover the hole in the bottom of your pot. If you don't have screening, you can also use a coffee filter or a piece of paper towel.
If your container is very large (and I'm talking huge), you can fill the bottom third with clean plastic containers, soda bottles or anything that will take up some space, but won't impede water flow. This can save you money on potting soil and make your container lighter. If you do fill the bottom,separate your soil from your filler by cutting plastic screening and putting it over your filler before you add potting soil. This trick makes clean up at the end of the season much easier.
You can also fill the bottom of your pot with a great product called Better Than Rocks.