My keywords ranking are changing quite frequently ,even i am working on it regularly to improve their ranking in SERP. please tell me how to stabilized them.
I don't think you can because that's how the search engine result pages (SERP) function. It's hard to predict where it goes meaning you don't know when or where your ranking will be. It's kind a like the stock market and it's hard to pinpoint where you are heading.
The only thing I can think of is just keep posting and share your company website info on social networking sites. Focus on Contents and SEO and doing backlinks.
It depends if you want to focus on long tail or short tail keywords: long tail means you have TONS and TONS of keywords, and each one gets a little traffic; short tail means you have a small handful of keywords that are really important to you. When going that route, you want to really focus on keyword density and that type of thing. I don't have super experience with this because DaniWeb is entirely user-generated content so I don't really have control over keyword density on the pages (outside of the header/footer/navigation).