You just keep on doing, as it is fluctuating. Also, you should not forget that competitors are out there and even they are doing the same probably with the similar strategies or much better than what you are doing. Could you tell us what all you have done and how much so that I could tell you accordingly?
If you want your keyword at top positions then first need is that you should observe about people's demand that what people enter higher for their query so as they enter query your , matching words related to query will be show in top with the site.
If you want to stable your keyword rank then first you have to make quality and unique meta tags of your sites then after make quality content with 2 to 3 % keyword density for you website pages and promote your site on high rank Forum, Press Release, Article and Blog sites. For this process you can stable you raking in google
analysis yours competitors site.SEO works for search engine often getting updated.so you have to work according to that.post quality content.promote your site in social networks.use dynamic xml sitemap.