I had pretty good luck running some white vinegar through the iron while rubbing the crusts off against an old towel. Then run some clear water through a couple of times.
This has the added benefit of clearing your sinuses.
Vinegar (Acetic Acid) will do it, but a solution of Citric Acid will do it better–Citric Acid complexes better with the calcium in the deposits (so sayeth the chemist that keeps our distilling machine clean).
Coming from a guy that irons just about all of his clothes, I can refer you to the local grocery store shelves. The toxic chemicals that break up the mineral deposits are usually found in the same area as sizing and/or starch. I will metion, however, that with the very low prices of steam-irons these days (about $12), you’re probably just better off buying a new iron every few months. In regards to distilled water…you’ll want to check with the manufacturer of the iron if possible. Some irons actually recommend that you use tap water vs. distilled because the particulate matter found in the liquid actually helps the iron work better…