I'm a newbee so haven't mixed a ton, but I actually use spring water for this. My tap water is really hard, and for the extra 80 cents for a gallon of good water I think it's worth it. I use the heat and then mix (off the heat) method, I don't quite boil it, maybe hit 160-170. Most of the sugar disolves, but there's always some stubborn little bits. I let it cool in the pan with a lid slightly that's slightly open, and by the time the whole thing gets to room temp all the sugar is dissolved.