Find the places that do bring you good results and keep advertising there. If something does not work for you, don't use it any more.
The website itself, also referred to a blogs, forums, twitter, facebook, linkedin, and the like. Places where people can simply interact with each other freely.
SEO free traffic.. but most people don't know how to do SEO corectly so they can't get consistant traffic to their site. So the biggest problem becomes how can one get targeted traffic to ones site..
KEYWORDS is the name of the game, no matter where or how used. Classified ads using keywords are very effective on high ranking sites. On page or off, in print or verbal, keywords are what wins in the end. Choose your words carefully and use variations of your keywords.
Most USED? hmmm... I am thinking it used to be backlinking, but since Google has cracked down on a lot of that stuff, I am not sure what folks are doing, except maybe social bookmarking, tweeting etc...? CONTENT is king! That is what I do and it works for me.
Writing, of course! Whether you're writing articles for article marketing, content for your pages or posting to your blog (or guest posting on relevant blogs, etc.) it's all about content! SEO, strong back links and QUALITY content = SUCCESS!!
I think SEO is probably the most famous. It gets mentioned like it is an easy thing to do. I'll be honest, it was what got me started. I figured I would be able to build small adsense sites for things "nobody else was doing" and then rank them and flip them. Lol, I left a lot of backlinks and tried my best with SEO and am finally ranking but since moved on to other options.