Given that Google now provides over 75% of all Internet search traffic, the last possible thing any site owner would want is to be banned from the Google index!
With countless search engine marketing techniques being employed these days, and contrasting advice available all over the 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 the 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 the following tips.
Spam
Never, ever spam. This involves sending a large amount of unsolicited mail via your domain mail server. Although the 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 the 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 the 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 the links on separate pages.
Cloaking
Seen by many SEO specialists as probably the thing most likely to result in a Google ban, cloaking involves creating one page designed specifically for the search engines, and another which will appear for the user. This is search engine manipulation at its worst.
Selling PageRank
Some sites have gone so far as to sell PageRank - i.e. selling links on highly ranked pages. You can sell links (i.e. advertising), but you cannot sell links for the stated purpose of increasing Google Pagerank.
Doorway Pages
A few years back, doorway (or gateway) pages were as common as normal pages - these are usually small pages, crammed full of keywords, designed solely for the purpose of gaining high rankings. They usually look awful and a site which uses a lot of doorway pages is likely to be penalised.
Excessive Cross-Linking
Some webmasters create multiple site, often with identical or similar content. They are then heavily linked together with the sole purpose of increasing Pagerank. Not recommended. If you have several sites, inter-linking is fine, but in moderation as with all search engine marketing techniques.
Submitting multiple URL's from the same site
An example would be for a webmaster to submit mysite.com and mysite.com/index.html to the Google database, thereby essentially trying to get two search results for the same page. If Google doesn't find a new site quickly anway, just submit your index page ONCE!
SEO Software
Don't use unauthorised computer programs to submit pages, check rankings, etc. Such programs consume computing resources and violate Google's terms of service. Google does not recommend the use of products that send automatic or programmatic queries to Google.
If you follow these guidelines and steer clear of excessive search engine marketing techniques, you should be fine. Just remember to create a site with is faithful to the product or service you are providing information on and you should be fine. Concentrate on exchanging quality links with similar sites and don't get too obsessed with your Google Pagerank. Quality, returning traffic is the goal.
For Google's webmaster guidelines, visit http://www.google.com/webmasters/guidelines.html
James Leckie is editor of http://www.bytestart.co.uk - the small business portal and http://www.trafficgeneration.com - the web promotion portal.