We almost always have some sort of practice / sticker / review chart that my daughters use to track practice.
This is the current chart, and the one I use most often. I list their review pieces and the chart helps us visually keep track of what pieces need to be played each day. The simple act of putting a sticker on the chart is motivation enough for them to play the piece.
I looked around a little bit and found some free, printable charts on-line: