Sara wants to be a princess, but not in the whole fancy gowns and marrying Prince William kind of way. No, she wants to be the kind of princess who has fine manners and a compassionate view towards those who are less fortunate (nice dresses are a bonus):
"If I WAS a princess—a real princess," she murmured, "I could scatter largess to the populace..." (5.79)
To Sara, princesses represent steadfastness of character. Remember how she imagines herself as a British soldier when her father leaves at the beginning of the book? Well, gradually, she seems to move from comparing herself to a soldier to imagining herself as a princess. We get the sense that this change is part of growing up, too. Rather than identifying with male models of heroism, she starts imagining what it means to be a heroic woman.
And her answer? Princess. In other words, Sara's mantra seems to be: "What would a princess do?"
She should probably just go ahead and turn it into a bumper sticker already.