With countless search engine marketing techniques being employed these days, and contrasting advice available all over web, it is well worth ensuring that you do not 'over optimise' your site or use any techniques which will result in Google penalising your site.

Although main rule would be to create a site which caters for your audience, provides quality content and contains meta information which is faithful to your site content, you should always optimise your site code to aid in your search ranking efforts, but this should be done in moderation, and in line with following tips.

Spam

Never, ever spam. This involves sending a large amount of unsolicited mail via your domain mail server. Although legality of mass mailing is a grey area, sites which do this deserve to be banned by every search engine.

Link Farming

Link Farms, or Free-for-all links pages exist solely to help listed sites gain higher search engine rankings. These are bad neighbourhoods and are frowned upon by major search engines. Obviously you can't control which sites link back to you, but you can ensure you don't link to link farms.

Excessive Links

Now that so many webmasters are more obsessed with their Google Pagerank than amount of quality traffic they receive, link pages are fuller than ever. You should try not to place too many outbound links on a single page. If you do need to link to 100 or more sites, place links on separate pages.

Cloaking

Have you ever searched for something on Google and noticed that a website has a double listing – a main listing and a second listing indented – and wondered "Why?" And then wondered, "How?"

Google lists up to two pages from a domain for any given search. Most likely, you already have two pages listed for your major search terms. Your pages might be listed at #14 and #456, in which case they don't show up together.

When two pages from same domain show up on same SERP (search engine results page), Google groups them together. If your two pages are listed at #11 and #18, they will be grouped on second SERP...assuming Google's default of 10 results per page.

However, you can change your default number of results in advanced search preferences: http://www.google.com/advanced_search .

Or you can make a one-time change by simply adding "num=17&" or "num=7&" (without quotation marks) after "search?" in SERP URL. Here is an example of how a 9-result page URL looks: http://www.google.com/search?num=9&hl=en&ie=UTF-8&q=SEO+tips&btnG=Search&meta=

One of my client sites ( http://www.dotcom-monitor.com ) delivers web site and network monitoring. As I write, it is ranked #3 at Google for "web site monitoring". One page ( http://www.dotcom-monitor.com/web-site-monitoring.asp ) specifically about web site monitoring is ranked #4 for that term.

Actually, using technique above, I learn that second page is really ranked #7. How? I set num=7& and see a double listing. I set num=6& and double listing goes away, bumped to second page.

Why does your competition merit a double listing at top of Google's rankings and you don't?

Google wants to deliver results that are important and relevant. In a search for "apples", your site on baking might have a very relevant page and it might even be an important page (and hopefully a tasty one, too!). But your overall site is not as relevant to apples as some others. In a search for "baking", however, you probably have dozens of highly relevant pages, giving Google lots to choose from.

I should note in example above that most of my client's site is about web site monitoring, so it makes sense that two or more pages would rank very highly for such a term (except that Google will list only top two).