My favorite answer to this problem comes from "How to Cut a Cake: And Other Mathematical Conundrums." The solution in the book was that "fair" means "no one has cause to complain." It doesn't work in the case here, since one party wants to divide the pie unevenly, but if you were trying to make even cuts, it works. The algorithm was: 1. Make a cut from the center to the edge. 2. Have one person hold the knife over that cut, 3. Slowly rotate the knife (or the pie) at, say, a few degrees per second. 4. At any time, any person (including the one holding the knife) can say "cut." A cut is made there, and the speaker gets the thus-cut piece.

At the end, anyone who thinks they got too little (meaning, someone else got too much) could have said "cut" before that other person's cut got too big.