Heat rises, cold air sinks, so partially close some vents on the lower floors during the summer and partially close some upper floor ones if it's too hot upstairs in the winter. Be sure to check for any dampers, often labled "winter" and "summer" near your air handler. Also change your disposable air filters more like once a month rather than the three months they say. Unless you don't run A/C and heat much...like my friend from France! Click on image for link.
