There is one thing I see nobody mention here....your current stats....
If you have a good statistics program you should look into your current searches. People have a way of telling you what they are looking for.
Use your existing search terms (you will most likely see that it is not single keywords anymore but rather search phrases) and incorporate that into your marketing and
SEO strategy.
With marketing I mean blogging about the most searched for terms etc.
It is all about content - your should use a combination of your existing search terms and the results from keyword research.