Don’t try to do too much at once. Clean a few shelves first so you’ll have places to move things to and make sure you have good enough light. Have a HEPA vacuum nearby. Set up guidelines in advance or after you look at a few things, like: accessioned material on one shelf, materials that you own but don’t have numbers, on another, and a separate space for more recent donations that you’re not sure you want to keep. After you work for a couple of hours, perhaps, stop sorting and deal with each group separately – take digital photos of the un-numbered pieces and put temporary numbers in the photos so you don’t have to keep referring to the objects. Make sure you don’t accession donations until you are sure they really belong in your collection. Once you have some items that you have questions about, write down the questions and try to set guidelines to make decision consistent: what are the alternatives, who shares the decision-making, etc. You want to group things so that you can make decisions for a whole group of things at once rather than making them one at a time.