If yours is a relationship filled with strife and stress and ill will, then you'll spend a great deal of time managing that relationship rather than managing your PTO toward success. Not only does that hurt your group results; it also makes your leadership job fairly miserable. Nobody volunteers for PTO leadership out of a desire to fight with the principal.
If you have a great PTO-principal relationship, appreciate it and work to keep it strong. If you don't, then now is the right time of year to start making it better. Have you sat down for a discussion with your principal about what she would like your PTO to accomplish this year? Another great question is "What has our PTO or former PTO leadership done in the past that we can do better this year?" Here you're looking for how the principal prefers to communicate, what kind of advance notice and approval she likes to have on things, etc.
Finally, my best tip is to do what you can to make your principal look good. Even if your principal doesn't lift a finger to help with the fall fair, it does you no harm (and potentially a lot of good) to publicly thank her for her support of your efforts. As much as we'd all love our principals to be superheroes -- great at all parts of their job -- some don't excel at the involvement/public relations aspects of the position. We can either get really frustrated about that or unselfishly take steps to help the principal fill that gap.
Whether you have a great relationship or you're struggling, we have some excellent resources on proper care and feeding of your principal. Check out "Make the Principal Your Partner" and "Negotiating With the Principal" for starters.