The SEO do's
Choose the right keywords - Choosing the right keywords is probably the most important step you can take. it may mean the difference of ever being found in a search engine result or not.
Get Backlinks - Backlinks are the heart of the surfing world. people jump from one page to another by following backlinks. Make sure your backlinks are coming into your site to more than just your home page. After all, you have more to offer the world than that, right?
Website Maintenance - Just because you built it, does not mean that you are done. Good search engine optimization is a continuous process. Constant updates and additions to a website are a must.
Remember the META Title Tag - The META Title tag is prime campground for keyword placement. It is the link that people see to your website in a search engine result, and it is one of the very first things a search engine spider will run across on your site.
Create a Site Map - If your site is large enough that not all of your pages can be linked to from the home page, then you should consider a site map. It consolidates all of your pages into one easy to find location, and it allows a search engine spider to find a link to all of your pages.
Validate your html - Having html code that is in compliance with W3C not only looks better, but it is much easier for search engine spiders to read. You can use the html validator on W3C's website to check your code, and make sure that you don't have errors that may turn a spider away.
The SEO Don'ts
Use Meaningless Keywords - Your keywords should be relevant to your page. If I were searching for green squares, and your website was in the list of results, I would be pretty ticked when I went to your site, only to find out that your trying to sell me a dishwasher. It won't help you make a sale, and it can be looked upon as spam by search engines.
Spam Keywords - Let's face it. I hate spam, you hate spam, and so do search engines. So why do it? Spamming keywords on a page makes no sense to the reader, and a search engine will frown upon your site, if it determines that you are spamming. It may even get your site banned by a search engine. It is usually devastating to a business that can never be listed in a search engine again.
Too Many Graphics - We know that search engines see images on a page as just empty holes in the html. Imagine if your page is just filled with img tags. What do you think that looks like to a search engine? Yep, a whole lotta holes. There are other reasons not to use too many. Page load times are increased, and many people are still on dial-up connections. Some users are on computers that don't display images. What does a page look like to a person that can't see the images? Yep, a whole lotta holes.
Hide Text or Links - If you want your site banned from a search engine, this is a good way to do it. Old tricks of using text the same color as the background, just to try and stuff in keywords, is now being caught by search engines. They catch it easily, and they may ban for it.
Use Duplicate Pages - My only question is why? Why would someone do this? I can only speculate that they are just too lazy to write content for a new page. besides, search engines are now watching for this, and your site will be penalized for it.
Multiple Pop-up Windows - Oh how I hate websites that do this. In fact I despise these type of sites. I can only hope that the fleas of 10,000 camels make a nest in the shorts of a person that builds these sites to do this. What is the point of going through all of the trouble to get a visitor to your site, only to make them hat5e you this much?