How do I get even temperatures in my two-story house?
If you have forced air, running just the blower fan to recirculate the air may help. Most thermostats have a way to manually set the fan to "auto" or "on", and you can also get thermostats where you can program the fan to come on at regular intervals. 

Beyond that, there are changes you can make with varying levels of difficulty and cost:


Plant trees or install blinds to shade windows where sun is coming in and heating the room more than the rest of the house 
Check for poor insulation around the house (this can be done with an IR sensor or camera).
Check for and fix drafts and air leakage 
Install a multi-zone heating system. This requires some potentially major work to the house -- varying from installing dampers and separate controls to installing a second HVAC system -- but it would give you complete control over heating/cooling in two (or more) areas of the house.