How to maintain stability in keyword ranking of website?

To main your ranking, you should not do any illegal ways of optimizing like black hat SEO or spamming because it will cause dropping your page rank. As possible when you do optimizing you can do blog commenting and forum posting which can help gain traffic to your website. Make sure that your comments can help the readers to get their impressions. And of course your ranking is depend on your keyword that you use, you should provide relevant keywords.