Keyword Density and Your Rankings
Keywords should be chosen with care. Use your keywords as often as you can while still letting your page "read" well. The number of times your keyword comes up in your copy compared to other words is called keyword density.
Depending on who you ask, what search engine you are dealing with, and the phase of the moon, you will get a different answer from every SEO expert on what the right keyword density should be.
Don't try and "trick" the search engines. You'll almost always lose out in the end. Real content, written for real audiences, usually ends up outranking sites that try and cheat the search engines.
Decide right now to play by the rules and when your competition begins to get banned, you'll still be up and running.
There are some important places to put your keywords. Take a look at blackbeltdomain.com. Can you guess what terms they're trying to rank highly on? That's right, they're trying for a high rank on black belt profiles. They did a very good job of it, too. Look at the page source (right click on the page and select "view source"). Take a look at their header tags, content, and the meta-tags.
It's a brand new site at the time of this writing and is topping out at number 9 in Yahoo after only a month.
I'm sure they kept keyword density in mind while they were writing the site. But they also kept the user in mind with informative menu options and clear text.
There is nothing wrong with viewing the source code on other pages to learn how they do it. As long as you don't steal code or copyrighted work it's OK to "look" for SEO training purposes.