Where keywords can be used

In order to ensure high search engine positioning, you need to make sure that you are using your web sites keywords (the ones that describe your products or services) in as many places as possible. Places you should be inserting keywords into are...

In the title tag: This is the most important place for you to use any keywords that are relevant to your site, make sure they're unique for each page of your site
In alt tags: These tags are meant to include text that will be displayed if the person browsing has their graphics turned off. Search engines will look at them and some do use it when deciding ranking
In the body text: This is what the visitor to your web site sees when reading one of your web pages. Make liberal use of your select keywords here, a ratio of about 3 to 8% should be about right i.e. for every 100 words of text mention your keywords between 3 and 8 times
In domain names: When selecting your domain name it's worth considering one that includes one or more of your keywords for higher relevancy. Some search engines evaluate your domain for keyword relevancy so extra points can be gained by having keywords included in your web site name. Any advantage you get will be slight and won't matter in all search engines. Northern Light and AltaVista seem to give a slight weighting to this in their algorithms
In page names: Use your keyword phrases in your page names. E.g. If one of your key phrases in that page was "top keywords", then name that page top_keywords.htm
In image names: If your trying to sell seo software and one of your images is of an seo software box then you could consider naming that image file seo_software.jpg
In keyword meta-tag: This tag was developed to enable search engines to get information about what your pages theme or content is about. Not all search engines use this tag any more because of abuse by webmasters, still worth utilising though. Get more information here about meta-tags
In your comment tags: You can put keywords here, but most search engines ignore them due to relevancy. The only penalty you will suffer is that of code bloat N.B. It's rumoured some search engines still consider comment tags
In the description tag: Search engines read this tag and so do people so put keywords in here that sell the benefits of your sites products and/or services
In the header tag: Understandably search engines give more prominence to text that is encapsulated within <H1> tags or <H2>, <H3> etc. Use this tag on important headings and subheadings that contain your keywords
In your link tags: Use keywords in the description that accompanies your link
In background images: If you use a background image give that a file name made up of your keywords e.g. search_engine_ranking.jpg would be a file name that would be applicable to a background picture on this site
In a noframes tag: Even if you aren't using frames

Know your keyword space

Some of the people in the Top-10 of your keyword-space are your competition, and some are your allies. Companies that make products that complement your own can be your best allies. Look at forging links with them, so at least you have the chance of getting some traffic from them being in the Top-10 for your chosen keyword. Also look for more allies on results after the Top-10. Penetrating into the Top-10 area in your Keyword-space is going to be your primary objective

It's worth occasionally going through your primary keywords on several of the major search engines. You'll get a valuable insight into who's hot and who's not. It is also worth trying subtle variations of your primary keywords to see how the results change

Testing your keywords

When it comes to benchmarking the success of your search engine optimisation campaign, evaluate your positions using a two word keyword phrase. Why two words in your test keyword phrase? You'll see meaningful results quicker on two words, you may never see results on one word, especially if the word is competitive e.g. software. Don't give up on aiming for a single keyword though. Instead, make sure one of the words in your benchmark phrase is also what you would use as your single-word target. Over time, you'll have opportunity to focus in on one of the two words as your primary target

Too many keywords

It is also important to discard potential keywords that are not likely to be used in a search. Properly done, you might end up with as few as 3 keywords from an initial list of 30 or more . If you use all of the words it will dilute the importance of each keyword, so select well. In general, you will have a hard time keeping the list short if you include plurals and various combinations of words to maximize your frequency counts

Summary

Write captivating titles and descriptions weaving in your important keywords
Remember most engines are case sensitive, so you can use that to your advantage. "Tips" and "tips" are not considered the same thing. Using "Tips" (initial letter a capital) is best as you would be considered in a search if a user typed "tips" or "Tips". At least on most search engines
Create content-rich information pages (doorways into your site)
Use a site map and enable another keyword rich page for your site
Make sure that your important keyword-containing tags and text are toward the top of the page. Any technique that pushes the keyword-containing tags or text further down on the page is detrimental to search engine rankings.
Monitor what your competition is doing
Having too many keywords on a page will dilute the importance of any individual keyword or phrase. Come up with 2 or 3 unique keyword phrases for each major page in your site, that reflect that page's content
Try to use your main keywords in your links. Many of the engines determine what the best keywords for your site are by the words that are in your links
There are search engines that give greater relevance to text that is in a larger font size, bolded or has been encapsulated in <H1>, <H2>, <H3> tags etc.
Use keywords where ever you can, they're your friends

the importance of Page Titles