Here is a clever variation of a launcher which is self-supporting on a sturdy frame, sent by Tracy Bahr of Wisconsin, made for his cub scout pack. It's a little hard to see, but the red arrow points to the valve that the air pump hooks to.

Here is a link to an organization for young engineers in Britain where the students are doing interesting things with water rockets. Check out the video links when you scroll down on this page http://www.youngeng.org/index.asp?page=1075

Correspondent Daniel Berry says that a kind of Mexican soda bottle works better than the kind sold in the U.S. I know some serious rocketeers intentionally heat and re-form bottles to be longer, and here it's already done for you.