Did you know that Google Analytics forgets the first traffic source through which a user has entered a website? This means that if the user clicks on a Google Adwords' ad, adds this website to his bookmarks, and then converts after two days, the conversion will be attributed to direct traffic.
To solve this, Google Analytics offers the "no override" attribute, which allows keeping the first referral, by making persistent the _utmz cookie for traffic sources. The nooverride attribute can be used in two ways:
Through a little addition to the Google Analytics code, you can keep the cookie of any user, independently of its traffic source. You just need to add this line before 'trackPageView' in the Google Analytics main code:
This is the best option to use the "nooverride" attribute, but curiously, it's also the less known. If you apply this option, the (direct) traffic will diminish drastically, and you'll have a greater control over all traffic sources, especially SEO.
Its main advantage is that it allows keeping the cookie for organic search traffic, which would be impossible in the next option. I use this first option in my website, as you can confirm by checking the HTML source code.
You can also tag URLs with "no override", if want to apply this attribute only to some traffic sources, or if you don't have influence in the main Google Analytics code. It's enough to add "&utm_nooverride=1" at the end of the URLs you want to tag:
Since the "no override" attribute is not compatible with automatic tagging, you should change "manually" all the URLs, though you can also use an Adwords script or an Excel macro. To measure SEO, this option is not useful, since you can't tag URLs inside Google's SERP.