How to recycle filament material from printed parts?
There are a few options.


Machines are available which grind the used plastic into fine pieces, melt it down, and extrude it as filament to be reused. Filabot is perhaps the most well known.
Depending on where you live the local recycling programs may accept PLA or ABS. They will then shred it and melt it down for reuse.
PLA is bio-degradable so you can put it in the compost.
I put scrap ABS in acetone which results in a slurry which can be used as a glue to attach ABS parts, fix cracks, and hold parts to the bed.