I also live in an old home (over 100 years!) where the second floor is significantly warmer than the first floor. This is the third summer we've been in the house, and the first summer that it's relatively comfortable upstairs. Each year we seem to learn a new trick.
The first year we were in the house, we replaced all the windows. The old ones were probably almost as old as the house itself, and they provided almost no insulation. Installing brand new windows was a huge help in keeping the cool air in during the summer and out during the winter. (It also cut down on the loud noise of freight trains passing by outside.)
Last year, we had an insulation company come add insulation to our attic. It turned out that there was almost none to begin with! They gave us the right amount, and also added a second vent from the attic to the roof.
This year, we purchased a second window air conditioner to use upstairs. With an AC unit running in the bedrooms at each end of our upstairs hallway, the second floor is actually quite comfortable. In fact, I even caught myself saying to my husband, sitting downstairs, that I was going to "go upstairs where it's cooler." The new AC was about $170, and well worth it. It also has an energy saver feature so you can specify a temperature and it will only run until that temp is reached, and then shut off. (The first AC we had was purchased used from the previous owners of the house for about $80 - it's not that old, but they've come a long way in the last few years.)
Good luck, and keep cool!