Would you try for SEO, Adwords, or other ad programs? Why? How cost effective is it?
It does depend on the site to some extent. Organic SEO is the best way to get lasting traffic - for any website - so make sure you've covered the basics (titles, headings and so on).
You may want to do some press releases to try and generate some buzz and get natural links. Social media like Facebook/Twitter is good for this - get friends/family/colleagues to become fans/follow your brand.
Google Adwords is a good idea for an initial boost, particularly for e-commerce websites. If you sell a particular product or type of product, bid on those keywords. You can of course continue after the initial outlay if it proves very successful (e.g. for every dollar you spend you get more than a dollar's profit from sales).
Finally, don't forget the real world! If you have a brick-and-mortar presence, do in-store advertising. Or take out ads in newspapers, magazines etc.
For highest value traffic - find blogs that are relevant to the topic of your web site and try to get them to write about you, participate in forums in related areas, find people who tweet about the topic and contact them, do guest post - whatever it takes to get your self out there.
For longer term traffic - create a blog to build your own following, use social media and get as many friends/followers has possible, create a mailing list and get people to sign up
For sustainable long term traffic - do SEO (this is mostly about publishing content - and that goes well with point 2).
For an expensive quick less-high-quality traffic use AdWords and other PPC advertising.
From my expiration learning to use AdWords is difficult, all the default settings are optimized to make Google more money not to get you good traffic - and until you learn all the major pitfalls you will spend a lot of money for nothing (also after you get over the major pitfalls you must continually tweak your settings to keep everything working).
For me AdWords was not cost effective and a single post on a mid-traffic blog (that recommended my product without me contacting them!) got me more sales than anything I've done.
For immediate impact, I can't say enough about social media and link exchanging--albeit legit link exchanging with real, honest sites, not bombarding forums with off-topic link drivel.
My most successful clients get more traffic from social media (twitter/facebook/linkedin) than they do from Google. It's not a measure of not being good in SEO, because they do a great job. They just do a BETTER job on the social media sites with multiple tweets and posts daily and and incredible amount of link sharing that's tied to social posts. A similar, but often overlooked means is by participation in relevant "enthusiast" sites related to the site's content. For example, a programmer might go through Stack Overflow and post on Smashing Magazine.
Adwords and SEO are great too. Just don't count on SEO for a brand new site being anything in the beginning unless you're in a busy category like Relators, etc. Adwords are immediate, and imho can be considered "purchased SEO" They work great.
All these options require daily attention, monitoring, and focus. AdWords in particular will decrease in the number of clicks they send you if you'd don't constantly monitor and tweak your account. Monitor Google Analytics to know where people are coming from, where they're going, and what they like. Tweak unpopular pages or content. If Google is low, focus on Google. Invite feedback about what would make the site better....people will tell you if you ask.
- You have meta tag with appropriate description and keywords
- You are using robots.txt
- You placed sitemap.xml on proper place.
Check my answer Why isn't my website in Google search results?
Moreover, do you ever write articles on other sites? Are you submitting your site contents & sitemap regularly to search engines? Are you tlling people about your site by submitting their conetnts to social bookmarkiing & social networking sites?
There is a list of questions.... just check whether every question have "yes" in answer.