The U.S. Department of Energy recommends you install interior rather than exterior storm windows. Despite their being more effective at keeping the cold out, the DOE says that they're easier to install, remove and keep clean. The downside is that interior storm windows won't protect the exterior of old windows from the elements.
Examine the corners of the windows before buying. In general, you want mitered or overlapping corners, not ones where you can see through the joints. For even greater efficiency, buy low-E storm windows.
The DOE recommends buying glass-pane storm windows for greater visibility, though plastic may be more affordable.