Fertilizer comes in many different forms. Liquid forms are usually a suspension or solution of fertilizer in water. The solids may settle out over time, so you may need to shake the bottle to dissolve or suspend the fertilizer if it has been sitting around for a couple seasons. Also, be sure to keep the container from freezing. If you have granular or water-soluble crystals of fertilizer, keep them dry. It's not enough to protect them from rain, snow, or other precipitation; you also need to keep the humidity low.
In high humidity, granules or crystals cake together and may form a solid block. If you keep them dry, though, they can be stored indefinitely. Keep the fertilizer in its original bag or container so there's no question about what it is. If the bag or canister has been opened, wrap the remaining fertilizer tightly in plastic to exclude moisture.