Since I do not hold regular lessons in December and hold Christmas Camp instead I like to hold a little practice contest during that month to encourage self motivated practicing. My prize for the top practicer is usually an iTunes gift card. I have done goodie bags filled of musical themed goodies, including Staple’s Easy Button as well.
In addition I come up with a secret number of minutes that I feel that all students are capable of doing that month. I don’t tell them what the secret number is until after it is over, but all students who reach the minimum that I expect, they get a special treat for doing so. (Usually a candy bar)
You can download my holiday practice sheet where they keep track of their minutes.