The following are a list of mistakes can ensure that your site maintains a low ranking with the search engines. Avoid at all costs.
Specifying no title for your page
I cannot stress how important the title of a web page is. Failing to specify a descriptive, keyword optimized title will do untold damage to your ranking with the search engines. It is the equivalent to owning a shop and boarding up its windows. Ideally each page on your site should have a unique, content-specific title.
Excessive use of images or Flash animation on a page
If your web page has plenty of nice-looking graphics and eye-popping Flash animation and not a lot of textual content it may indeed look nice but have you ever considered what how the search engines might see it.
Search engines thrive on textual content, scavenging as much text as they can but unfortunately they cannot understand images or Flash animations like we can and so will find nothing of real value on your page. Try to balance your page so that the textual content is given priority and that any images or animations are used only when needed. Also it is a good idea to attach some text to an image by using its ALT tag as search engines use this text when determining rank.
Complicated menu systems
If you must use a complicated menu system be sure to provide a site map that is clearly accessible from the homepage of your site and contains only textual links to your pages. This ensures that even if the spider cannot understand your menu system that it will be able to find the pages on your site.