You shouldn't have to freeze your tail off every night just to go camping. First, understand that staying warm in a sleeping bag is all about heating the air in the bag, and then trapping that heated air inside the bag. When your sleeping bag is directly on the ground, you mash the air out of the bag's fill, and you let a lot of cold come in from underneath you. The first suggestion is to sleep with a pad under your sleeping bag, not simply a plastic ground cloth.
Do what you can to warm your body before you get in the bag. Twenty jumping jacks before you hop in the bag may keep you warm for hours longer, because your body will better warm the air in the bag. Once you're in the bag, strip off all non-essential clothes so your body heat can be used to warm air, not clothing. And of course, don't sleep with wet clothes. You will also sleep warmer if you wear a hat, stay hydrated, wear warm wool socks, and fluff your sleeping bag before you go to bed to restore its loft.