When we parent-proof all three of them at the end of the year, we use coins. It’s pretty motivating when they get to put a coin on top of each fact instead of just a sticker. (Equates to ~$16 for dimes, ~$8 for nickels, $1.50 for pennies.)
I only have one child who will be officially memory-master proofing all the way through the director proof. With the other two, I still go through the process of proofing them as a parent - they're my little Memory Masters in Training. To know a bit more about what a Memory Master is, click here (though that post was written last year!).Download a copy of the Memory Work Sticker Book here.