if you look arround this site you will find endless suggestions on how to fix that very thing.
the best rule of thumb is insulation (covering the top and sides with a blanket) and ventilation (making sure the heat and moisture can't escape). a rubbermade will definitely be easier to manage. i use a glass tank and its fine. of course atlanta doesn't have that much snow, but temps and humidity are definitely an issue.
you are going to need to tell us what settup you currently have (as specific as you can get with sizes wattages and current temps and so on) and i know that people will start chiming in and giving advise