In these days of unpredictability in the search engine world,
I really wonder how or what can companies do to maintain their top rankings.
What should be done to maintain site's ranking?
Hope to get some useful answers.

Why maintain? Why not move up? Keep building links, higher page rank and quality links if possible. You should always try to improve your rankings, not "maintaining".

As Google is continuously changing its algorithm of displaying result in SERP, once you get on the top it became little difficult to maintain the ranking as so many factor works together and you have to maintain the quality of back links along with getting new one.

I think there are lots of way to maintains your site ranking,Here i define some good step to maintain site's ranking.

1.Post fresh blog/site constantly.
2.Use authority sites.
3.Create contextual backlink.
4.Get involved in social media

I think you are talking about keyword ranking, Keyword ranking doesn't happen instant, needs lot of hard work and smart work especially the off page activities like directory submissions, social bookmarking, article submissions, press release, blog commenting and forum commenting.

HI
maintain the quality , good quality is the only way to maintain the rank.

I f you want to maintain your site's ranking you should do quality back linking on regular basis.

Get quality links from the product based websites then automatically your page rank will increase.

To maintain consistent website ranking, you need to pay attention to your main keywords and related keywords.