If you work within an office, it is wise to setup a simple system for your own work-related information; however, I do not advise co-mingling this with your personal information (doctor, bills, kid’s information). Instead, setup a similar system at home.
Likewise, if you own your own small business, I do recommend keeping two different systems: one for your personal affair and a separate system for business related information (contacts, accounts, contracts, invoices, etc.).
I have seen a lot of variations (color coded, priority ranked, etc.) with added complexity that is really unnecessary. I truly recommend keeping these simple as possible. An alphabetical filing system works best as it does not require an elaborate index to discover where something is filed. In addition, if someone else needs to find something, such as an assistant or significant other, a simple alphabetical filing system is intuitive for anyone to use without any special training.
Simple filing systems help avoid messy desks and data loss. After something has been processed, it should either be filed to the simple reference filing system or it should be thrown away (assuming it is no longer needed).