Search Engine Optimization -- the process of making a site "search engine-friendly" — is probably the most important aspect of website design. Many, many commercial websites are designed and set up by people who know little or nothing about search engine optimization — how to give the search engines what they want to see when they index your site. This long page contains the information (or links to it) that they SHOULD know about optimizing a website for the search engines.
The most important thing to know about search engines is that SEARCH ENGINES ONLY INDEX TEXT. Make that your mantra while designing and optimizing your website. They don't index images, and they don't index Flash well. Although there are some exceptions to that rule, as search engines are becoming more intelligent and more able to index files such as PDFs and the captions of graphic files, and some of the text in a Flash file — for all practical purposes just remember that rule: Search engines only index text and you won't go too far wrong in search engine optimization.
There are a lot of small things you can do when designing or re-designing a site to get better treatment in the search engines — and every little bit helps in the end result. There is no one magic thing you can do to get top placement at a search engine for your website. But you can do a bunch of small things that will add up to excellent placement in the search engines for the key words you select.