I did a science fair project on crystals once. I used alum (available at a pharmacy..people use it for pickling) and tested whether distilled versus tap water, and growing at room temperature versus refrigerator, made any difference in crystal formation quality/quantity. I think the results were all similar, but the room temperature/tap water (surprisingly to me) had slightly larger and better crystals than the others.
like notanumber said, you have to supersaturate the water. I used baby food jars full of water, and sat them in a saucepan which was also filled with water, and boiled them. once the water's boiling, add your solid (alum, salt, sugar, etc) a little at a time. stir it a little...carefully, don't burn yourself. keep adding it until the water won't dissolve any more solid - the point of supersaturation. Take them off the stove. crystal formation occurs when the water evaporates and the impurities - i.e. the solid you added - are left behind. so you should start seeing crystals right away as steam continues to come off the water. let them sit for a few days and as the water evaporates, the crystals will get bigger.