Thread gets brittle and breaks when it gets old. Storing thread in airtight containers, such as Ziploc bags, will keep it from drying out as quickly. Thread exposed to sunlight and heat will break down more quickly.
Thread can be stored in drawers, containers, or on purchased thread racks. If you use racks, make a dust cover out of fabric or plastic. If using drawers or containers, organize them by color for easy comparison and place spools so that color is obvious without having to pick up every spool.