Using wax on clothes is an old technique for waterproofing gear, but it's not reserved for just backpacking supplies. The Art of Manliness shows how to wax anything — from hats to jeans — to give them a bit of water resistance.
Typically speaking, wax is meant for canvas clothes, but it works on just about anything. You'll lose breathability but gain water resistance. All you need is a bit of wax, a hair dryer and some clothes you want to waterproof. Just heat everything up with the dryer, apply the wax, heat it again and dry. Head over to The Art of Manliness for a full guide as well as pictures of different results.
How to Wax Your Own Clothing and Gear [The Art of Manliness]