Once you have clean urls (search engine friendly), meta descriptions, <title> tags, header tags and ensured you have the correct keyword density on your pages what else is there to do?
Siloing (categorisation) is the process of organising either physically or via use of links your site content into logical subject areas so that you communicate a focused message about the subject areas you cover on your site to the search engines.
There are some useful articles on this subject here
It is possible to prevent valuable page rank weighting from being passed on to pages which you do not need or want to benefit. Examples of these might be your terms and conditions, user login, contact forms. I have found a really good article about how to use the rel=”nofollow” tag to achieve this.
Most web agencies put a credit to themselves in the footer of their client sites. This is a good practice but they often target terms which they already perform well on, their company name. Instead you might want to read this article about alternative anchor text to select.
Use an advanced google search to locate Drupal Websites which allow users to comment with links. The important thing is to search within the context of the term you are optimising for.
Google are now indexing 350 characters from meta descriptions for their search results when users provide more than three keywords. This means you have double the space to get your message accross and can get more keywords into the text. It is also helpful because you can send a more meaningful message to searchers.
It is easily overlooked but it is essential to get your robots.txt file validated. You can use an online validation service.
I found this really good article which cuts of the waffle and explains how to use title tags effectively and actually explains properly why you shouldand should't do certain things without adding any SEO mystique (bull).
Drupal is excellent with the search engines, a good start is to employ the SEO site Checklist module which lists most of the essential techniques on setting up seo with Drupal.
Drupal has an issue for SEO in that it can appear to search engines to present duplicate content. e.g
There is a module called Global redirect which you can install, however you can alternatively edit your htaccess file and the robots.txt so you don't have to support yet another module.
The Drupal Path Redirect Module is very useful when you are migrating an old site to Drupal. The earch engines will have your previous site indexed and be directing traffic to the legacy pages. You do not want to lose this traffic by sending them to a 404 page not found error. This is where Path Redirect comes in.