As others have noted the two primary things to avoid are sunlight and heat. I would say anything over 70 degrees for extended periods can lead to detrimental aging.
Beyond that it will depend on the type of beer and what your goals are. If you have a big imperial stout or alcohol beer which you want to age and mature you're going to want to keep it in a cool dark place, upright, but not refrigerated. Refrigeration will greatly slow the aging process. If you want to keep your hoppier and/or lighter beers fresh and crisp refrigeration will help by slowing the deterioration of the fresh flavors.
I'm sure others can chime in more scientifically as to oxidation and the other things that will occur under different circumstances.