“How to evaluate and improve my SEO without tanking the great rankings and traffic I’ve enjoyed for years by accident.”
There are two sides the the SEO equation: on-site, and off-site. Both are important in order to improve your ranking.
For on-site SEO, your first step is to determine how you’re doing right now. Google has some tools to help with this. If you haven’t already signed up for Google’s WebMaster Tools, you should do so. There, you can find out how Google views your site: what keywords they associate with you (whether you’re specifying keywords for your pages or not), your geographic target area and more. You can also specify a site map and see if Google is having any problems crawling your site.
Using Google Analytics you can see what people are searching for and how they are finding you and also find out if you’re getting a lot of bounces. If you’re already seeing some success on certain pages, knowing how customers find you can help you in focusing your content on those pages so more people can find you.
While any changes to your site can effect your rankings in the short term, having more focused keywords and better content will help you in the long term.
For off-site SEO, you should focus on getting links to your site. While all links are good, some are better than others. You should focus on links from sites that have a higher Page Rank than you do and are preferably discussing the same topics. If you sell flowers, it doesn’t do you much good to have a link from a site that sells cars. Getting a link from a site that talks about flower arranging or growing flowers is better. A few links from well-respected sites that are in your same industry will count for a lot more than a hundred low quality links from random sites.