There is a veil of mystery surrounding SEO tactics. There are many modules out there that aid in improving your site's exposure. The easiest path for a beginning is to use SEO Tools. It basically gives you a list of prerequesites, and gives you some easy out of the box SEO.
If you have a deeper understanding and you want to do things for yourself, I recommend the SEO Checklist module. This one simply gives you a checklist tool and lots of recommendations on improving your SEO. There are a handful I found quite useful, and others I chose not to follow. It's the perfect aid for more advanced users.
These are most important items I found out of all the recommendations:
- Clean URLs
- Redirect/Global Redirect modules
- Meta Tags module
- XML Sitemaps
- Registering, verifying, and submitting sitemaps to search engines
If your site allows user contributed content, then preventing spam is also important. Mollom comes highly recommended for this.