Paper clips can abrade or scratch the surfaces of prints or negatives. Pressure sensitive tapes usually contains acids which can accelerate the deterioration process. Any kind of ink also contains acids. Fingerprints on prints or negatives create physical damage from the oils and acids in human skin. Also, the intense light and heat from copiers are detrimental to photographs.
High humidity is most harmful and high temperatures accelerate the deterioration.
This applies equally to negatives and prints. Handling the original is NOT a wise thing to do. On plywood, masonite or any composition board. Original photos are one of a kind and should be professionally mounted. Your original photo should be properly protected when viewed by others.