Buy a million email mailing list for $10 and spam everyone!
(jokes)
PPC is good but does cost money.
I like to find quality forums to join and interact with. Well, ones that let you have a signature.... But I DO NOT SPAM. I honestly try and post quality info and not just respond with "nice post" or "good info".
Back to PPC, an additional advantage of using PPC at the beginning is that you will cause Google or Yahoo to crawl your site faster if you use their PPC. So $10 for a quick spider is not too bad.
Of course blogging and the use of the social bookmarking sites help out.
Creating a video for youtube can also work.
I tried forcing registration by gun point but the return traffic was lacking.
There is no one way I believe, but rather you have to use a mixture of all of the methods.
my 2 cents
HCG