Site's ranking is falling. How to stop it??

[brian]

The easiest and simplest way to stop and acquire your prior rank is to update your that two pages. Add some content to them if you add images then its better, add some tags, customize your webpage according competitive analysis to the pages which are ranking top now etc.

Frank has well put it. I totally agree with him. I will just add a bit.
Creating new links can also give you boost just don't make link to the pages which you want to rank well in the SERP's. Build links to you site's homepage this will make the impression to the search engines that you are not trying to manipulate the rankings but naturally building them.