Here are a few pages you might want to consider. I browsed them when I developed my first launcher design.
My current rocket design involves two 2-liter PET bottles with broken cds for fins. The launcher is made of 1/2" pvc pipe, cable ties, cut out portions of PET bottles, adhesive, some rubber bands, and a length of rope.
This one averages the height of a 15-storey building. I've also added a parachute system for recovery.
The 4 sites below should jumpstart your experiments
http://www.geocities.com/wrgarage/
http://homes.managesoft.com.au/~cjh/rockets/
http://www.smoke.com.au/~ic/water-rocket.html
http://www.geocities.com/CapeCanaveral/7914/how.htm