Well I know that people have an Issue with writing Documentation for anything but over at Joel on Software he has a rather good (Old but good) Series on Functional Specifications. The first in the series is: http://www.joelonsoftware.com/articles/fog0000000036.html

I've gone through this and he does put a good call as to why we should do it and I agree it can save so much time. The site to point to this originally is: http://www.mojofat.com/tutorial/index.html again another great site on how to write an FS / Who to write it and why.