You can post the url at forums like you did , but this risks to result in a bad reputation of spammer.
So all depends of your niche, and the content of your site. It also depends if you mean paid promotion (ads, affiliates, etc...).
So you can promote your site on social networks, by exchanging links with other sites with complementary content, if you have registered members, you can run a newsletter, to inform them of the news related to your site, etc etc...
What's your budget? If you have enough money to throw at it you can become famous worldwide almost over night (within a week at any rate, for about 7 minutes of fame). This costs more than chump change, pocket change, or even a year's salary as a VP at a Fortune 500.
Otherwise, you promote your site by doing good work, don't offend the beneficial bots, have content UNIQUE AND COMPELLING that is not easy to steal and replicate, and the users who want to see that content, and a server able to handle the potential traffic with speed and accuracy. (Might involve going to the cloud)
Real world webmasters realize it might take 1-2 years to build a presence on the web, and another pair of years to start seeing profits (if the intention is a commercial project).
You do social media, newspapers (local), tv (local), radio (local, and by far the best option), and joining your local and regional business alliances (if your site fits), hand shakes, meeting folks, and sell, sell, sell, at every opportunity.
The web is a very noisy place these days. To be heard you need the biggest megaphone you can afford (and NOT offend the search engines) and work at it 24/7/365. That includes all holidays!
Know that sounds like a mountain range of difficulty, but you actually conquer the mole hills first, then move up in scope! It can be done on a budget of Micky D's per week ... just takes a heck of a lot longer, and stoic patience, and unwavering commitment.