This product works well -->

http://www.atsko.com/shop/catalog/SILICONE-WATER-GUARD-12-fl-oz-aerosol-p-16174.html

I've seen it in our local hardware store and you can easily find it other places online (Amazon etc.)

One spray can is sufficient for any poncho or tent. Be sure to thoroughly wipe down the surface with a wet towel (and let dry) before applying. Spray on and wipe in well with a paper towel. Let dry. Repeat. Two coats will do wonders for restoring water repellency.

-H