Accessibility and SEO
Images and Meta Information
Keywords
It's something of a truism, but is often ignored: you won't get listed on any search engine for words that aren't on your page.
More than that, if the words are important to your page, they should be in the prominent places on your page, such as the first paragraph and the <title> and <h1> tags, and subheadings.
(Caveat: You can be listed for words that other sites use to link to you see this wikipedia article on 'google bombing').
Titles and Headings
The titles and headings are among the most important things on your page. This is from anyone's point of view. Your <title> tag and <h1> tag should describe what the page is about, and, ideally be the same.
The balance and positioning of the keywords in your titles are equally important. Conventional wisdom has it that your most important keywords should come first in your titles, and that only the first three words have any real weight.
Thus, if your page is about 'Orange Medium Widgets' sold by WidgetCo, your optimum title would be 'Orange Medium Widgets: WidgetCo', or even: 'Widgets, Orange Medium: WidgetCo'. However, if you think your customers would find greater benefit in reading 'Welcome to WidgetCo'; so be it. It's a balancing act.
Images
Just as with creating an accessible site, it's never a good idea with search engine spiders to rely on images to convey important information. If your headings and sub-headings are prettied up by being images (it's perfectly acceptable practice to nest images in <h1> tags), it's worth noting that search engines tend to give less weight to alt attributes.
Summing Up
Of course, this is all heuristics and reverse engineering. Google don't give out their algorithms workings and look don't like people developing sites for the sole purpose of gaining high rankings. They want and are committed to catalogueing the internet and its pages so that the most relevant and useful results are returned first.
Equally, you don't necessarily have to write your page content for the search engines. You can ruin some pretty good copy by shoving keywords in the wrong place. You can choose to ignore search engines, or balance keyword placement with wordsmithery.
What you can do is to use these techniques not to give yourself a sneaky leg up the rankings, but in order to ensure the information you have on your site is presented in such a way as to make categorising your site easier.
