Site content should actually contain the keywords you choose… actually a better way to think about it is that your keywords should be chosen based on your site content
It is important to drive home a very important point – forcing keywords into places that do not belong will be clearly evident to indexers, crawlers, and your readers. Be sure your keywords fit naturally within their locations.
Think “specific keyword phrases” not “keywords”. Due to the extreme amount of competition for general terms in the search engines, if your keyword phrases are too general it is very unlikely you will rank well in the search engines. You stand a far better chance to rank well for specific phrases where there is less competition. The resulting traffic, since it is more highly targeted, should also be much higher quality too!
Keywords should appear within the first paragraph on your page
Keywords should also appear in H1, H2, H3, etc. Tags.
Only one H1 tag is recommended on a page. Note that changing the style of your header tags (H1, H2, etc) will not affect rankings
Bold and strong tags emphasize the importance of words to crawlers too
Keyword density should be between 3-10% of the content on your page
Keyword Proximity is also important. Be sure to place keywords close to each other
Keep in mind that the order in which the terms are typed will affect the search results