Many SEO experts today recognize that on-page SEO is not the key to high search engine rankings. That may be so, but if your pages are not properly optimized, then you're at least partially wrecking the work you've done elsewhere. Better yet, for the most part you will not need to re-optimize your pages later – once you do it, you're done. As you add new pages, you need to build them with optimization in mind.
Overall, there are four types of optimization: Keywords, tags and titles, links, and content. Keep in mind that keywords are critical in all SEO techniques, so you really have to nail your keyword strategy first, and then implement it through all your other optimizations.
Keyword use is the most critical element of your site, but they are a little delicate. Since your site is built around them, they should be expressed whenever possible. But because you do not want duplicate content, they should not be used more than necessary. Clear, huh!
Seriously, though. You're keyword is what your site is about. Primary phrases should be expressed in the URL, and others in your titles, tags and headings. It should also show up in your content. But at the lower levels, do not force the keywords in.
Probably the most overlooked factor is page titling and title tagging. Search engines read your titles when they crawl your site, so make them count. Since every page has a unique purpose and unique content, it should have a unique title and title tag.
Other critical factors are title tag length and order. First, think about length. Many webmasters use the title tag to express the link relationships. The title tag then reads "level 1 | level 2 | level 3 | …" This is unnecessary. The URL is a unique description of the page location, so just stick to the title of the page.
In thinking about length first, we may have solved the other potential problem: keyword weight. That's an important factor in page ranking. If your word is 10 characters and your title is 80 characters, you've demoted your keyword to 12% of your title. Do not do that! Just use the title, which will be more like 50% -100% keyword.
As everyone knows, links are critical. First, for search engines to index all your pages, they must link from another page on your site. That should be easy enough. The next thing is to pull your links, anchor text, and keywords together. You're using the phrase "rubber balls" to sell rubber balls? Then use the anchor text "Rubber Balls" to link to a page "http …. / rubberballs." You might even check the title tag make sure that also says "rubber balls."
Content is by no means least important. Your page titled "rubber balls" should be about rubber balls! You should have some variant of the phrase "rubber balls" your H tags as well, like "yellow rubber balls" or "super bouncy rubber balls". Last but not least, you will not be able to boost your rankings by boring the text of your page with ridiculous or unnatural keyword usage. Keep it down to less than 5%.
That was a quick discussion, but you see the pattern. Implement your key-phrase and word strategy into your on-page SEO, and carry it through to every level of your site. Do not pass up any feature (tags, links, H headings etc) to describe your site well and uniquely characterize the content. Good luck!