This error below made me realize I hadn't compiled the rules/laws/principles of proper and effective keyword usage for SEO:
1. One main keyword (phrase) per page. Now, sufficiently long, you'll get other combinations of keywords which link to this page, but you are going for one particular long-tail niche (forget one-word keywords, these have been sewn up long ago and are maintained by huge sites with massive link-love coming their way.
Reason? Your page is known (reputation) by its in-bound links. Whatever they say is on the page is really more important that what is on that page - but you want all inbound links to say the same keyword as you optimize the page for. If you work on getting prime search engine real estate (top 5 positions - above the fold), then figure for a keyword "scatterbrained nitwit", all or most of your inbound links should have the words "scatterbrained nitwit" in their text link. While you can get "scatterbrainednitwit.com" to help out, it's not essential. (But note: having "scatterbrained-nitwit.com" can get you penalized, as spammers wasted that approach years ago.)
Two or more keyword phrases will confuse search engines and give you lower rankings. Your pages should be like you talk - one subject at a time. Don't talk or write in non-sequiturs. If you do have to bring up another topic, it's perfectly OK to link to a separate page on this new topic. Gives you another keyword phrase you can "dominate".
2. Use your meta-tags for theme words, not keywords. Most of the biggest search engines don't rely on meta-tags (thank spammers for that one, too...), but Google uses them to check the "theme" of the page, ie. does your content match what you are talking about?
If you put your keyword phrases in your meta-tags, unscrupulous spammers will scan these to find what you are talking about or trying to get search engine real-estate with and work to beat you to the punch. Just what you need - more competition.
The old tools which harvest meta tag information are good now for finding theme words - what that page should be talking about. The reason Google looks for theme words is also to help them figure where to list your page. Most words have several distinct meanings. Searching for "Apple" the computer company shouldn't give you results on "apple" the fruit. Help them out - use meta tags for repeating theme words on your page.
3. General current use of Keywords:
a. Page title
b. H1 heading
c. Once in opening paragraph - which should be emphasized with bold (or possibly italic - but only in san-serif fonts, so it remains legible).
d. Generally, no where else on the page, or at max - no greater than 5% of that text copy - again, thank the spammers for this rule.
e. The rest of the page is composed of theme words and articles (an, a, the, those, that, etc.) and somewhat meaningless words which hold the sentence together (conjunctions and stuff like and, or, with, as well as of, in, by, etc.) But these common words aren't theme words and so shouldn't show up on your meta-tags.
This above is the result of several studies into page optimization, namely through Michael Campbell, Dr. Andy Williams, SEO2020, and some others.
While the rest of this below post seems fairly accurate, the below is not. (P.S. I blog this to give you all possible data, but also because their site has no particularly apparent way to add a comment.)
Website Magazine : Keyword Research - A Foundation for Local SEO:
"Focus on one or two keywords or phrases per page – and use them in the title, subheads, meta tags, and copy of the page. Also use keyword variations in the text to enhance the keyword’s relevance.
As an example, let’s say your business is called Las Vegas Floral Boutique. You could use your business’s name as one keyword phrase and ‘Las Vegas flowers’ as another. Since most people will be using ‘Las Vegas flowers’ rather than your business name in their searches, mention Las Vegas Floral Boutique occasionally but focus more on the more generic term and use variations like ‘Las Vegas flower shop’, ‘flower shop Las Vegas’, ‘affordable Vegas flowers”, etc."