Content: learn your users - then teach them
The best SEO technique of all is to deliver relevant content to a clearly-defined audience. But before you do that, you need to do some research. The first thing you can do is learn more about how your visitors are using your current site, and, in particular, the pages they are most interested in and the keywords they are using to find you.
A really great way to do that is to create a Google Analytics account (www.google.com/analytics). Analytics is free and powerful, offering complete statistical analysis for each and every subpage under the account. You can use it to search keywords, unique visits, bounce rates, average pages per visit and so on - it really is a breeze to use. Another great stats tool is StatCounter (www.statcounter.com). StatCounter is a little simpler than Google Analytics, but can be more useful for gathering snapshots of data at a glance.
Now, thinking about the content, remember that the average web user is not a specialist, (meaning that even if your site is about a relatively technical subject, you can benefit from using fewer technical terms. Check the keyword statistics for your website visitors (remember you have two sets of data at your disposal here: the keywords visitors are using to find your site and the open access raw search data that search engines like Google make available - be sure to check out Google’s external keyword tool, which is a great source of free keyword information). You’ll find lots of synonyms and misspellings, but remember the most popular few keywords are the ones you need to concentrate on.
However, there is a “but”: don’t rule out any of the the keywords in your data. You can give a keyword more weight on one page, but if possible use a synonym on another, separate page. This will increase the size of the “target” your website offers to searchers in your topic space.
Also, on the web misspelling is not much of a crime. People do it all the time and it is organic. Google helps to fix it, but you can always create explanatory redirecting page just in case. For instance there are 154 indexed misspellings of “chainsaw”.