Craters are hard to control - but the best way I've found is to quick-set the icing using a dehydrator or a fan, as others have mentioned. Certain shapes (small, angular ones) seem to be especially prone to them no matter the icing thickness (I have tried the gamut). The quick-setting tactic won't cure craters 100-percent (or at least it hasn't for me), but it improves the situation to about 90-percent crater-free!
I once quick-set cookies in my oven, but its lowest temp is 150F and so I found the drying harder to control there. (The cookie icing can quickly get too hot and over-expand or even crack in the oven.) I now set my dehydrator at the lowest setting (95F) and hardly have to watch them. Usually 10 minutes in it is enough to quick-set the top of the icing and prevent craters, though this time also depends on the thickness of the icing and the size of the space you're trying to set.
Interested to hear what others have to say as well.