The Paterson 120/220mm Negative Filing System is the most practical way to store negatives. Allows filing of negatives and contact sheet together.
It is based around a smart ring binder which contains 25 glassine sheets designed to hold negatives in strips. Each 120 sheet holds up to 4 strips of negatives.

Paterson's refill sheet for this filing system is quite costly. If you are looking for the refill sheet, you may want to consider the Glassine sheets (which is made from the same material in the Paterson file), available here.