The problem with ranking in the search engines and trying to stay in the same place is that the search engines change their algorithms and how they rank sites and pages all the time. One day you might be in position 1, the next day in position 8.
This is a struggle all webmasters have. One thing to consider is all of the sites vying for the same keywords, and the search engine needs to constantly decide which site to put in which position in their results.
The best thing to do is to continue to add fresh content to your site, optimize the content with keywords and meta tags, and also work on gathering more backlinks as this is a very important part of how the search engine ranks pages.
We have a series of SEO Tutorials which will step you through various ways to optimize your site for the search engines; here is a link: SEO Tutorials.