How does one keep waterproof pants waterproof without using special “hi-tech” cleaning products?

jillian m asked: I have owned some in the past and washed them as infrequently as I could stand, but they still lost their waterproofing. I am sensitive to chemicals and don’t want to buy the expensive “hi-tech” products. Is there a simple cheap way to clean them? For example, using baking soda or vinegar [...]