Find a spot out of the sun for storage. If your garage has windows, look for places that the sun doesn't hit at all during the day.
Submerge any waterproof items in water. This will eliminate the chance of the items being damaged by heat.
Use a cooler padded with sand for non-waterproof items. You can keep the sand in re-sealable plastic storage bags to prevent corrosion. The sand will insulate against the heat.
Use an army ammunition box for all other items. The metal casing is designed to keep heat out of live ammunition, so should protect most of your stored material.
Increase the ventilation in your garage. Open a window if you have one, or buy a small, portable fan and turn it on for an hour every day.