I can't comment directly to any specific pharmaceutical as I'm not a doctor or pharmacist, I am a chemist...just so you know my background and where I'm coming from. I'll try to keep this simple, but if you'd like more detail just ask.
The most common ways chemicals degrade either by oxidation or reaction with water (either from the air or sometimes the drug itself is sold as a hydrated form). The rate of degradation is directly related to the concentration of the oxygen/water and the temperature. Thus just like food, keep it dry, O2 free, and cold (as cold as you can).
The problem with keeping drugs in the freezer is that if you don't create a completely sealed environment, you will get MORE water in there than if you kept it at room temp.
One thing I should mention though is plastic is NOT a good barrier to keeping things totally oxygen and water free. It's an OK barrier, but O2 (and water to some extent) can and WILL slowly diffuse through plastic. Glass is good as is metal. A stainless steel (or aluminum) threaded tube with threaded metal caps and some teflon tape (very important!) is probably one of the best containers the lay-man could make. Put your drugs in there along with a water absorber and an oxygen absorber, screw the cap on and you're in business!