The best way to not lose ranking is to "not" build links too fast, especially for new website. And watch when you get high PR links for high PR pages in the beginning, that will definitely through up some red flags.

The best way to avoid lose ranking is to build links wit consistency and regularity. Do not try to get 1000 links a day and for the next 5 days you are doing nothing. Keep it consistent.

It is simple ...do not spam, do not put link whare you can't site.

The best and simplest answer for this is a statement that I heard from my friend and he said "Nowadays after all the new Google updates, all you need to have is a 100% perfect in-page optimization".

Get excellent quality and powerful back links from excellent pr websites then you will not drop you standing, i think standing is rely on your back links therefor always try to get more and more back links.

SEO is a process which should be be continuously going on now a days their are many competitors around the world and they will be working hard to get those ranking, so you have to build quality links always it will maintain your ranking and your website will be tough competitor for other companies. if you will stop working one your website keywords get on top then their is high chances of getting those keywords affected.

First of all find out why your rank is falling.. normally it may fall for poor quality backlinks or duplicate content.
so, try to get some good quality backlinks thorough unique content and high PR directory/SBM/ forum site.
however, in on page- you should ensure informative and unique content.

There are many methods you can increase the scenario. In the SEO boards, they will discuss with you.