In terms of all the time you spend on a particular SEO Campaign, 80% of this time should be spent on finding and building links to your website. Google is an off-site search engine, this essentially means that its rankings are based on a whole range of off-site factors including how many sites link to your site, how authoritative the linking sites are, how popular the linking sites are, whether or not the links are no followed, where the links are positioned on the page and what anchor text is used in the link - all of these factors will contribute to how prominently you feature within the rankings around your target keywords in Google. The reasoning behind this is due to Larry Page and Sergey Brin and their correct assertion that websites and pages with the most authoritative links pointing in can generally be regarded as the most authoritative around a particular subject and as such, they should be returned more prominently whenever a user types in a relevant keyword.
The main thing you need to keep in mind when link building is trying to find pages which will transfer some authority or relevance to your site whenever they provide a link to your website. Google's ranking factors are based on the democracy of the web - the more valuable the content is then theoretically, the more likely people are to link to it. The important thing to remember here is that Google ranks pages not websites - the homepage will generally be the most authoritative page on a website but it needn't be that way, it's merely a result of the fact that most people link to the homepage. It's also important to avoid placing too much emphasis on the importance of PageRank - while this is still a ranking factor, a link from a highly authoritative site like the BBC is, needless to say, a lot more valuable than a link from a PageRank 2 directory. Always keep this in mind when it comes to building links to your site, one good link can be more valuable than one-hundred sub-standard links.