It's best to write copy for the user and for search engines NOT for SE's solely. These days you do not need to stuff keywords into your pages in order to rank higher. You can include your most important keyword phrases in certain spots that will increase your chances of ranking under those keywords. The exact density is not as important as having the right distribution of keywords.
Start at the top of your pages, using Alt text within your logo and header images (have your logo link back to your homepage). Include your keywords for that page in your Title near the beginning. Include the key word phrase in your description 1-3 times.
Use page headings with your keywords in them (<H1> <h2> etc.)
Use keywords in other image Alt text, and hypertext links to your internal and external page links.
Use the keywords within the body text and bold text (do not make them all bold and do not stuff the text full of keywords).
You want to think of your page as an hourglass shape; distribute many keywords towards the top, fewer in the middle and many at the bottom.
Include your keywords within inbound hypertext links (use variations of words and phrases so they are not all the same like Paz said).
If you cover all of those places your keyword density and distribution throughout the page will be correct for SE's and easy for users to understand.
