Not so sure about the upstairs/downstairs issue (we have a ranch style house although we do have a basement) but there are a few things that I've noticed about uneven heat distribution in our place. One, depending on the type of heat you have (we have forced-air, gas furnace) it seems to be partly a matter of resistance--like if there are three vents, the one or two closer to the furnace/more directly in a line will have more air (either hot or cold depending on whether it's furnace or ac) coming out and the room will get hotter/colder. When I've blocked that off it seems to force air to come out before. One way I've done this is to physically take the vent cover off and cover it with something like aluminum foil and put it back on, to block the heat from coming out in that room. It then tends to force it back and more warm air will come out the open vent for the other room that you're trying to warm up.
You might also see if there is some other reason that the downstairs is too cold, like maybe there are air leaks with doors or windows and you could add some inexpensive weather stripping or plastic or even put a draft blocker at the base of the door.