I saw someone else ask this question recently but it was closed as spam before many decent answers appeared because they included links to their site.
I am however interested in genuine answers to this on a more general basis.
Avoiding talking about specific websites, what techniques and practises can people recommend to raising the level of quality traffic that a website receives?
[I'll post some of my ideas, but I'm interested to read what more experienced web developers can suggest]
As with any business proposition you must have good content (provide a good service, useful information, or a good product), and then make sure enough people know that you have good content. If your content is something that relates to particular services or products people look for, you need to make sure your site is optimised for search engines to index it by common descriptions of those services. Google's SEO guide is a good place to start for how to do this. If your website sells second hand cars, don't forget to include the words "second hand cars" on your page somewhere, it sounds obvious, but it's an easy mistake to make, particularly if you use a lot a graphics.
Secondly, you should consider advertising. You need to get the message out to people that your site exists and what type of content it hosts. There are all sorts of forms of advertising ranging from the relatively cheap (paid for links in Google searches) to the very expensive (TV adverts). Obviously the type of advertising that will work for you very much depends on the type of business you have, the profile of your customers and the amount of raw cash you have available to spend on it.
Finally, don't underestimate the power of social networking. Particularly if your site provides something that would be popular with the general public. Make a Facebook fan page for your brand, create a twitter account, blog about it, etc. Anything to raise public awareness of your brand. (Do however be careful not to 'spam' people. Too much invasive or unwanted advertising can have a negative effect and damage your brand image)
You mention good content. However what is "good" content is still unanswered? In my opinion it should be frequently updated and new material should be added. To get and keep users attention there should be a way to participate (like stackoverflow, feedback etc) and to keep track of all those changes (RSS-Feeds/email notification). Make all the information available to YOUR audience and don't make them suffer. What does it mean? Don't force them to use anything special (Silverlight/Flash/Javascript?!), don't popup anything, don't bug them with surveys and first and foremost deliver valid pages and deliver them fast!
Not exactly my opinion, but here are a couple other common things: With your keywords and content you should fill a niche, because everything else is already taken and you will not get high ratings on google. It helps if the site/domain is already a couple of years old, so don't drop or change domains too fast! Put yourself on every map and in every free service out there, but don't try and do some link farming thing. Search engines will punish you. Most (?) search engines don't understand CSS so you can add additional text to your site and make it more attractive for them (and hide it from the users via display:none), however don't exaggerate! Put your most important text/message to the TOP of the page.
SEO, high pagerank (by having high-pagerank sites pointing to you). Ads on google search results. Having something to offer of course (good information, fun, selling good stuff).