I like to look at the base of the boolits and make sure I have a nice square edge. A little frost is ok if that's where you get the best fill out. Some guys use a small manicurists fan to cool the sprue faster and keep the mold from getting too hot. The longer the boolits are in the mold the more it heats up. Pausing for 10 or 15 seconds after you drop your boolits before closing your mold up can help keep your mold from getting too hot.