Don’t have time to do a complete overhaul of your site? Not sure what SEO is speculation and what stuff works? Here’s the bare minimum to give your site a boost.
Use plain text as much as possible; navigation links especially. Search engines don’t read images or Flash.
Get links pointing to your site, especially to the content pages, preferably non-reciprocated with your keywords as the anchor text. The best links are from relevant hub or authority sites. Just ask! The worst that will happen is they say “no” rudely.
Put your most important content first. Then support it.
Use as few words as possible. This may be counter-intuitive but attention spans are short online (even for search engines). The fewer words you use the more power the words you do use have.
Include the page title and make sure it is about the page content, not your company. Including your URL is a complete waste.
Validate the code to be sure there are no ERRORS; warnings are ok for SEO (though still discouraged).
Add more content.
Get more links.
A quick glance at a page should tell you exactly what it’s about. If it’s unclear or has too many points of interest rework it.
This will ensure your pages get crawled properly and in the index. Adding more content and getting more backlinks will help you move up from here.
Of course there are hundreds of other techniques and the more competitive your industry the more of these techniques you may need to use but this is the foundation to build on.