Now that you talk about this of going to forums related to the specific niches,....I don't know If I am doing the wrong thing, but, As many of you have adviced, I've gone to some forums but I've noticed that If I don't want to be spotted as a SPAMER, I should spend a lot of time writing in that forum and after some weeks or months I can publicize my websites. I mean, it is very suspicious to register in a forum and after 5 or 6 post, I am already talking about "this amazing website I just visit the other day. you must visit it!" What is the RIGHT way of using Niche forums for promoting your website?
As some of the above people have said, using forum signatures is a great way to spread word about your new site. Try to find forums that relate to your own website and become a member of their community. Give and you will get back.
The best way would be to write multiple press release and submit them to as many press release website, and if your website is interesting, people will follow you, and if you're really lucky, maybe some bloggers will write on their blog about your website.