System needs to be simple and easy to maintain otherwise there is no point. I make use of a 2 drawer filing cabinet and use a colour coding system:
Drawer 1 – yellow files includes general and financial information
Drawer 2 – pink files includes marketing, projects and client information
You need to schedule time on a regular basis to do all your filing – I do mine on a weekly basis.
Remember, the simplest systems work the best!
Check out http://academyforonlinebusiness.com/about-us/we-recommend/ for details of what we use in our online business.