What to do to regain post's serp

You should first need to identify the reason why your rank decreased in the SERP. Consider checking your analytics stats and see if there’s some reason why traffic is down. Decrease in traffic usually occurs when your site have low quality links or duplicate content. In order to identify which backlinks are still working, you need to identify the bad ones. In the Google Webmaster Tools, look in the “Links To Your Site” section and you’ll see a list of the websites that link to your site along with the number of links that point your way from each website. After identifying the bad links, you can choose to delete, destroy or dilute the bad links by acquiring more quality natural links.

Quality backlinks means creating unique content containing your keyword to high quality sites. Diversify your keywords, as well. Build links from different types of authority websites with a mix of do-follow and no-follow blog posts, web 2.0s, wiki, social bookmarking, press releases, article directory submission, video, document sharing, etc.

In my case I never created any links except couple of blog comments, that too months ago. All links to my site are natural.

i´d add some more value - video, audio, a slideshare presentation and upload this material wherever i can. then i´d link from those sites to my post.
after that i´d add some links e.g. blog commenting to my page's portfolio