It seems to me like your target audience would be high school and college people. (College folk have nothing better to do between classes, hehe) So now that you know your target audience, how can you reach them? Find other forums targeting the same age bracket, which don't offer online games, and post post post! Link to your site in your forum signature ;)
I think a Press Release will help, also get in contact with some gaming webmasters with authority. If you can spark a few discussions here and there, you bound to catch some attention. Participate in gaming forums. You can also romote in youtube and in social networking sites...
Promoting a game site is also the same as promoting a site and
SEO will help you. Its a process of improving the volume and quality of traffic of a website on search engines. Just make sure to do both On-Page and Off-Page Optimization.
With online communities and forums you can build a reputation for your site Show them what you are made of and wow them with your range of expertise about the subject, with that you can build a reputation and build trust with the people in your expertise and knowledge.
And as Cscagal have siad, find your targeted audience is also essential!
The best way to promote and get traffic for online gaming sites is PPC campaigns from Google Adwords, Yahoo publisher network, MSN, Bidvertiser or Adbrite. You can also try buying cheap traffic from StumbleUpon or try putting banners or other ads on other famous online gaming sites.
Yes one of the best ways is by banner advertising, but Google and Yahoo are very expensive and don't really target users that effectively. Thus you end up with alot of click through's and little registrations to show for it.
Try a company called 'advertise your game' They specialize with browser based games and have good performance results to show.
Some advice for free promotion would be to contact all the gaming portals who deal with browser based games, there's A LOT I have been dealing with PBBG's for 3 years now.
Online community is a must since your game is online. You can go to schools or colleges too if you want to give brochures (though I think it's an old-fashioned marketing). You can always use social networking like facebook or friendster. Do you know how many fan page for Mafia War? A LOT!!!
What kind of online games? I would set up a blog and a squidoo page and post articles specifically on each particular game - segmenting the broad nice "online games" into individual niches, and of course point each blog and lens etc to a specific page on your site where you deal with that particular game.
Yeah. you need to participate in game related forums, add your ideas to that members. Joine groups related to gaming industry on social network like facebook, linkedin. it will bring you better chance to share you sites and relative ideas.