Keywords
Choose the right one. Use the Wordtraker or Google keyword tool together with the number of results on Google to find out what keywords are searched for and how many other websites are targeting them. Be realistic about what keywords your website can be ranked for, if you have a small website it will be difficult to rank well for a highly competitive keyword.

Frames
I personally advise people not to use them unless absolutely necessary. Frames tend to cause problems with search engines, bookmarks and so on, because frames don't fit the conceptual model of the web (every page corresponds to a single URL). Should you use them make sure you use the NOFRAMES tag and link every content page to your frameset index page.

Code
If your code is messy it could make it very difficult or even impossible for a search engine to properly see the content of the page.

Flash
At the moment only Google seems to be able to index Flash files, how much or how little content they see is unknown. Until search engine technology is able to handle your flash as standard then it would be advisable to avoid the use of it. You can use flash elements and embed them on a page, if you choose to have your page navigation in flash make sure you replicate it in straight html so that search engines can see it and follow it.

Dynamic URL
Although Google and Yahoo are able to crawl complicated URLs it is still advisable to keep your URLs simple and avoid the use of long query strings. Do not use session IDs in the URL as these can either create a 'spider trap' where the spider indexes the page over and over again or, at worst, your pages will not get indexed at all. If you do need to include parameters in the URL then limit them to two and the number of characters per parameter to ten or less. The best SEO solution for dynamic URLs is to use Mod-rewrite.

Title tag
The title tag is probably the most important thing for improving your website's search engine results. Make sure you include the chosen keywords in it and place them near the beginning of the title tag.

Meta tags
There are several possible meta tags that may be included in the head of a web page but the only ones worth using for search engine optimisation are the "Description meta tag" and the "Keywords meta tag".

Navigation
Search engines follows links in order to find and index pages and absolutely love straight HTML links. When creating the navigation for your site bear in mind that flash navigation, javascript navigation, area maps and drop down navigations will create problems with search engines.
If you use the above make sure you replicate the navigation as standard HTML links in the footer of your page.

Content
When writing the content speak the customers' language. Provide detailed information and make sure the content supports the subject. The keywords chosen for the titles should be found in the page content with a density of 4 to 8%, within a tag, in bold, in italics, in the link text (Google loves it) etc.

Links
If possible use text links, this will allow the use of keywords and are loved by Google. Don't use too many (Google recommends having no more than 100 on any given page) as it may lower the relevance of the page.