When you're finished soldering or storing the iron, before always leave a glob of solder on the working area of the tip. This protective coating prevents oxidation by sealing the tip from exposure to air and moisture.

Be sure to cover the entire working surface with a glob of solder, not just the very tip as depicted in the attached photo.

When you're ready to solder again, simply heat the iron, wipe off the excess solder with a brass or damp cellulose sponge, and re-tin the tip.