if you are already on the top of the search then you should keep doing what ever you are doing in past. Don't stop good work which help you to rank high. You need to study your competitors so you will always stand one step ahead from them.
To stay top on Google, you need to do very strong and effective SEO strategy for your website.
Build strong backlinks to your website. Also, keep working for your site daily. this is my suggestion for you.
Put Quality work on your site.
Like get niche related quality links by doing all SEO work of link exchange, Forums, Blog comments, etc. and update your site on daily basis for Google crawler.
and post quality useful articles and more.
By following the algorithms, measures & guidelines of search engines with honesty you can always survive on the top. This process is just time taking. Be patient & follow the updates of search engines.
You should update your web content time to time and should have only quality backlinks for your website. Don't try t adopt any of back hat technique like keyword stuffing, clocking and so on. Google can easily identify it. So just follow these techniques definitely thy will help you to stay on first page.
The most important thing is to read as much as you can about SEO techniques. Read articles , tips and tricks and any other material that you can find . After a little documenation about seo you should be able to answer to yourself to every question. It's better to know these things from your own reading experience as you may learn more than you think.