In the beginning no one downloaded MTG Forge because no one knew about it, so I began my publicity campaign (which I think of as advertising). The first public thing I did was to begin this blog. Starting out was difficult and I was only getting 2 or 3 views a day which was abysmal. I even thought about closing it down.
After I really understood what people wanted, just Magic and computer programming, I was able to focus and write articles that people enjoyed. I limited myself to only posting twice a week (for my sanity) and keeping the articles relatively short, 150-300 words.
The obvious first place to advertise is on Magic forums. So I posted some stuff on StarCityGames.com and was banned because I "advertised" too much. Now I understood that I need to advertise but in a gentle fashion, "Oh have you heard about this program that lets you play against the computer?" Another good place to advertise is the Wikipedia article on Magic: The Gathering. One day I casually added a paragraph about MTG Forge and I still get about 1/3 of my hits from Wikipedia.
Recently I advertised on cubedrafting.com. MTG Forge isn't really a cube but I posted about it anyways and 256 people read my post. (A cube is a special collection of cards that is specifically designed for drafting.)
Currently my blog gets around 150 views a day while is absolutely phenomenal but I still advertise a little in order to increase the number of people that know (and love) MTG Forge. :^)
StarCityGames.com did forgive me and they let me write an article about MTG Forge and recently Abe Sargent also wrote about MTG Forge (thanks for the advertising).