I guess for games, social networking sites will be able to play a huge role in terms of promotion. You can try promoting the game on your Facebook or MySpace profile. Add in a referral system if possible and reward your players for referring their friends. If you can afford advertising, buy some ads on various entertainment or browser game sites. I am sure the traffic there are fairly targeted for your niche.
Try submitting your site to social book marks such as digg, that will help promote it, also you can try to make a facebook app out of your game
another site to help you (both in SEO, and in promotion) is webgamefavorites.com You can register (and run for the Game of the Month) on webmasters.webgamefavorites.com
Is it a flash game? A facebook Application? or similar to Hattrick and Runescape? Maybe you could check this list of top gaming websites: Top Websites for Games and learn why these websites do gain so much traffic. Copy or improve their strategy. It worked for them and it might work for you.