Keeping my filament spools from getting knotted?
When I first set up my printer, the spool was able to spin freely. Filament was making loose coils on the outer layer of the spool. Snags happened frequently. I decided that not allowing the spool to spin faster than the rate draw of the filament might improve the way the filament was feeding. 

I put more physical distance between the spool and the machine. The filament stopped bunching up in loose coils on the surface of the spool, and the snags went away. The increased amount of slack between the machine and the spool seemed to absorb the "overshoot" of the filament draw and the spools became much better behaved. I also was able to run the spool down to the end more consistently than before.

Not everyone has extra room to use, and not all extruders are conveniently oriented, so moving the spool further away from the machine won't work for everyone. I believe a guide or some sort of dampener would serve the same function, if the underlying problem is the spool loosening up enough to where loops of filament get caught under others and then snag. What worked for me was to stop the spool from "racing ahead" of the rate of draw and loosening up enough to snag.