This fall I just started having my students keep practice books. I put a year's worth of practice charts in a paper folder and they are to color in an object for each day they practice. Most I created myself, but I borrowed a little from the free downloads on your site.
I wanted to find a way to reward students who practice the most, but then didn't want it to feel like a competition where anyone would feel excluded, even if they practiced a lot. Also, I know they things happen- people get sick, strings break, etc. So I set the criteria that the student must practice at least 20 days out of the month, and if they do, they will earn a spirit stick and each month has a different theme. Twenty days isn't my ideal, but it seems to be a reasonable goal. Some of the kids will have no trouble achieving this, but it is a challenge for some!
I'm just starting to give out the "I love Suzuki" sticks this week as a free starter to their collections so they can see what they're like. I think they're excited about it, even some of the teens who may not wish to show how cool they think it is. :) I even have an adult student of retiree age who is playing along with my incentive program. :)
I love this picture of my student who has practiced many many many days in a row!
Here is an example of what I gave out at my Spring solo recital as a gift/treat at the end.