Most of us who use gas cookers end up with numerous part filled canisters. I recently purchased an adapter that allowed me to empty one into another. This proved to be super useful and I emptied about twenty canisters to get three full ones!
This video explains the process.
I purchased my adapter from Dick Smith for around $20. Once my canisters were empty I punched a hole in the bottom with a nail and let any residue gas out. I then placed it in my recycle bin.