Having the kind of game that draws crowds is a dream for game designers. If people gather around your game and get excited to ply it, you obviously made something special. No matter how many advertisements and banners you post, the biggest thing to help grow your game into viral popularity is to make a game people want to talk about.
Make a game that makes a passerby want to stop and look at over another person's shoulder. If the player is expressing excitement, it'll make nearby people curious as to what the fuss is about. Give the player something to laugh about and want to tell to their friends later. you could even try to tickle a niche's funny bone and get them to spread it on forums.
The big picture here is to not forget that the players are the most important part of your game. Players are not just numbers and sales figures that stop mattering after you've taken all the money you can from them. They keep the game alive and running for as long as they choose to.
So give them a reason to choose to.