lets say i got good rank for desired keywords , what should i do to stay at top forever ... how to compete with peoples who try to secure first rank .... also what are the different ways my competitors can use to secure better place than me .
Just keep building good links and make sure your page stays better for the end user than your competitor's. It's as simple as that.
Yeah, basically what TOPS said. Just look up your keywords on google, look at the competing sites (top 5/10) and see what things are good about their site and maybe see what your "page strength" is compared to theirs (page strength tool at http://www.seomoz.org). Also, think to yourself WHY you're beating them (the ONE MAIN reason) and focus on that for a bit...
Also, now that your at the top for your main keyword, start branching out into other related phrases. Your added link building that focuses on some other phrases, whether those links point to your home page or internal pages, will also add to the link popularity of the entire site. But dont neglect your main keywords, continue some amount of link building to focus on that phrase too.
See where your competitor is getting their links & get a link from all those sources + keep building new links from newer sources. That too.
Very good advice! If your site is worse or even just as good as the competitors that won't cut it. you must be better than them.