Keyword Density and your rankings.
Keyword's should be chosen with care and you should use those keywords as often as you can ,and still "read" well. The amount 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, or the phase of the moon, you will get a different answer from every SEO expert on what the right density should be.
Don't try and "trick" the search engines. You almost always loose out in the end. Real content, written for real content 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 will be up and running.
Still there are some important spots to put your keywords. Take a look at blackbeltdomain.com. Can you guess what terms they are trying to rank highly on? That's right, they are trying to get ranked well for black belt profiles. They did a very good job of it too. Look at the page source (right click on the webpage 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 toping off at number 9 in Yahoo in 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 on other pages to learn how they do it. As long as you don't steal their code or copyrighted work it's ok to "look" for SEO training purposes.