Ultimately, the best approach to the widespread problem of temperature imbalance between floors of a multi-story house is to thermally and hermetically isolate the attic from the house interior as much as possible. Doing so will minimize the following drivers behind the temperature imbalance:
Summer: reverse stack effect, where sinking air caused by cooling it draws hot, humid outdoor air from the attic through multiple ceiling penetrations
Winter: stack effect, where rising air caused by heating it (and escaping into the attic through the same multiple ceiling penetrations) draws in cold, dry outdoor air through multiple penetrations/gaps/holes in the lower floor walls, windows, and etc.
If the envelope can't or won't be fixed, the only alternative is to improve the design of the HVAC to compensate for a crappy envelope design. If we insist on staying mired in the 20th century, when fossil fuel energy was cheap and abundant, with our thinking on how residential building envelopes should be built and perform, we're facing a potential situation where many homes may become unfeasable for their owners to remain comfortable year round, as energy costs continue climbing over time.
21st century approaches to residential structures should be more thoughtful design efforts toward the envelope AND the HVAC. But alas, we're mired in a "get 'r dun" culture, and until that changes we will continue seeing monumental stupidity in how we build and control residential building environments.