The keywords are relevant for the search engines in two places inside the site: the <Meta Keywords> Tag and the content of the site. When you write content keep in mind a very specific idea and choose your keywords before you start to write, that makes an important difference between creating an optimized site from scratch and the work needed for later optimization.
A research done by Entireweb says that 31% of people enter 2 word phrases into search engines, 25% of all users look for 3 word combinations and only about 19% of them try their luck with only a single word.
Do not choose a keyword to optimize your site for that you don’t have the slightest chance of ranking good with because of the fierce competition.
Do not choose a keyword that nobody looks for. Make a simple comparison: how many people searches for differential equations versus shoes versus porn (or sex). So use only generally popular keywords if you do not need targeted traffic.
Do not choose a keyword that does not relate strongly enough to your content.
Do not use words that may get your site filtered or banned from search engines.
Do not use images with file names or ALT tags (Alt attributes of IMG tag) that may get your site filtered or banned from search engines.
Use lots of relevant content, well laid out into separate pages.
For best results optimize one page for one keyword.
Do not post online half-finished sites.
The source is here.