There are many ways to build traffic in mainstream. The best is good ole organic SE traffic. Buying traffic is my second favorite traffic method followed by article traffic. Youtube traffic is also very good followed by other video sites and forums that are related to your niche.
From what I have learned in the past 6 months or so, to do very good keyword research before promoting something. Exact match domains are very good for targeting SE traffic.
Heres a video I made a while back about doing some keyword research.
http://pronichefinder.com/Expired_Domains/ Not very good but it will give you an idea on how to research keywords.
The way I drive traffic is by doing my research, Lets say I want to promote an anti virus program. I would use market samurai or googles keyword tool to get some keyword ideas on anti virus software. I would look for something with at least 1000 exact monthly searches or more. I would also look for lower competition, by putting quotes around the keyword in google. If the results are less than 20000 it would be a go for me. I would also look at the top three sites listed for that keyword and hope they are something I could outrank. Thne I would register an exact match domain as the keyword either .org .net or .com.
I would write a nice long presell type of article (500+ words) for the landing page of the site. Its important to have a very good presell article on the landing page of the site. You want it to mention the keywords of your site several times with a 2 to 3% keyword density. Most of the traffic is going to go to this landing page.
I would then do keyword research for articles. I make sure that the article would be closely related to anti virus and have a min of 100 or 200 exact monthly searches. I will also make sure the competition is below 20000. I try to write around ten articles when starting a new site out. I use wordpress for most my sites so I sticky the presell article to the homepage and list the articles below it.
Once all of the articles are indexed by google, I start the article marketing part. I have a program called article marketing robot that will submit my articles to hundreds of directories, but to start I just used a handful of article directories. I would create an author sig for my articles at the bottom and put the links back to my site using the keyword I want to rank my site for. Sometimes if my site is already ranking for a keyword I would put something like see the original article here using the article title as a backlink to the article page.
This will drive some traffic to your site, but not tons. The next step is to turn your articles into videos. You can simply read your article and throw some pretty background stuff in it and submit it to youtube or any other video sites.
This will get the traffic to start trickling in, but another advantage of article marketing is syndication. When you submit your article to the directories a lot of blog owners will come there looking for articles to put on thier blog. They will see yours and list it on their site hopefully. What I like to do is search the web for my articles monthly to see who is syndicating them. When I find someone syndicating them, I will contact them and ask if they would let me be a guest blogger for them. I tell them I will give them my new articles before any article directories will have them. For me, this has been one of my best sources of traffic besides google.
After going through all of this work you should get a decent placement in google for your keyword(s) and have some organic traffic along with article and video traffic. For me its all in the keyword research. You find that right buying keyword that has traffic and is easy to rank for.