If you want to get listed for secondary keywords (which most of everyone does), use them infrequently in the body of your post.
The theory is that the more times a keyword appears within a blog's web page, the more relevant the page is likely to be for someone searching those keywords.
But do not overdo this by repeating the same keywords over and over again. Google bots can find out if a keyword is too frequent on a page and might just remove your site from their index.
As outlined below, checking a keyword density using software is a smart idea. See the post on WebCEO in April 2007.