Changing pieces definitely helps. I try to work on a 2 or 3 pieces at a time so I can go back and forth between them without getting bored.
But last night, I tried working on 5 different pieces, and my interest and fun level grew by leaps and bounds. I just couldn't take my self away from the piano. For the first time, I practiced forover 2 hours. I usually get bored after half and hour, but from now on.