Myth One: Just Put Up Meta Tags to Get Your Ranking
I didn't want to put this one here at first because everybody else mentioned this in their articles, so I thought that it was an open secret. Then I met a businessman who optimizes his blog via writing key word meta tags and doesn't do much else as far as SEO. I told him to "try and read a bit more about SEO," then I cleaned up his tags for free and offered to help with linking (for a fee).
About Meta tags -- please DO write relevant keyword and description Meta tags into your code. It gives spiders something to cache in the database. Without them, you will be left with no description when listed in the search engines. If your key word Meta tags are irrelevant to your content you may get penalized. The search engines do not place any sort of premium on Meta tags for ranking; I have seen and ranked sites with absolutely NO META tags. This said, please add Meta tags, they make your site look neater in the SERPs.
Myth Two: Key Word Density is Everything
The idea that any one thing is everything is probably the most dangerous concept. Google's PageRank algorithm has over 200 factors it considers and MSN's Ranknet considers over 600 factors. Onsite factors on their own are just a start to SEO; off page factors are also important.
This said, content must be relevant; if you are optimizing for the right key words your key words appear more important when emboldened and in headers than when all over the place. Avoid key word stuffing and spamming, you will get penalized.
Myth Three: You Never Need to Redesign Your Site
I could summarize the above statement in one word: cheap! If you can't afford to redesign, say so. Don't say it's not necessary. Sometimes a site is just not search engine friendly; for example, you build an AJAX-powered site, and then call an SEO expert who tells you to move the text away from the AJAX windows. If you tell him that "that's not necessary" you just made his job a lot harder.
If your site is not search engine friendly, you can still optimize, but it makes the whole process just that much more difficult. Your navigation structure must be navigation friendly to spiders. Your copy MUST be changed if it is not relevant to your target audience. If you insist on unfriendly design then you should include a Pay Per Click program; you will need it.
Apart from SEO, redesign is necessary to improve your users' experience on your site. Forms, layout, interactivity, aesthetics and more all need to be updated as web standards and usability change and improve. You can't afford to assume it's all about SEO when your users get to your site and flee at the quickest opportunity.
Now that we have finished with the On Page Myths, let's look at some Off Page Myths in SEO.
Search Optimization 
