How I bought the most sought-after ad space on Earth for $20 — Recently, my book was advertised on a Times Square billboard, right between Forever 21 and McDonald’s. Pretty cool, right? Here’s how I did it. Blip Billboards lets you advertise all over the U.S. I used a site called Blip Billboards ($25 free credit). It allows you to advertise on billboards all over the U.S., paying per “blip,” or 15-second…