I use styrofoam packing peanuts (make sure they're the ones that don't dissolve). Some people use empty soda cans when they have a LOT of space to fill up; never tried that.

Read Al's post on soil and water in the container forum. Give them as much dirt as the pot will hold, but wick them to drain water from the layer at the bottom that refuses to drain. You'll see what I mean in the post. Or you could double pot them as he does, putting a smaller pot inside the larger one and filling the space between the two with dirt.

I read somewhere else on gardenweb a tip on filling pot. I put plastic milk or soda bottles at bottom. Then I put in old newspapers.