I used the plastic print file things until I noticed my negatives were getting scratched. Now I use 8 1/2 by 11 xerox paper folded in half, one negative to each sheet of paper. Just drop the negs in and put in a file folder or large envelope. The better quality paper says its acid free.
The paper may say it is acid free but there is still the question of whether it meets archival standards. If you value your negatives then only use archival sleeves and boxes. Anything else is a false economy.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.