If you're purchasing a very large shredder, the lid and shred mechanism may be heavy or cumbersome, so look for a side-loading or slide-out basket rather than a unit where you have to lift the top off to empty it.
Make sure to keep the wastebasket empty. For a shredder to work efficiently, the shredded paper needs to be able to drop freely into the wastebasket below.
Paper draws moisture out of it's surroundings, therfore, the shredder's teeth should be lubricated regularly to keep the machine operating smoothly.