Hi,
I'm not an SEO guru but I've done a good bit of promoting our own sites. Here's some of what we try to do:
- Links! This is the key for getting traffic. The more you have (generally) the higher you'll be listing in search engines. Just contact similar sites and ask to exchange links. It's a long boring job but it'll be worth it in the long run.
- Submit to directories such as Dmoz & Yahoo. Have a search in Google for free directory submitters. (submit with an email address you're happy to receive a lot of spam on!)
- Advertise on very targeted sites using an ad service like buysellads.com. (yes it costs but it can be worth it)
- Post in similar forums. We use Digital Point quite a lot but there are lots of forums out there that you can use.
- Give away something free via forums. Maybe offer a number of free premium accounts.
- Setup a Facebook and Twitter page.
We've had mixed success with Adwords, sometimes costing us a fair bit, so you need to be very specific about what keywords you use. If you can spend more time on building the organic traffic, it'll be better longer term.
Remember though that there's lots of competition. If it was easy then everyone would be setting up the next mediafire!
Regards,
Adam.