We've been using the following motivation method for over a year now. It's working out great.
The kids earn screen time by doing extra chores. (Some chores they are just expected to do as a member of the family-put away your own laundry, clean up your toys, keep your room clean, help set the table, etc.) Also, on school days they get a point once all of their schoolwork is done. These are written up on a whiteboard where it's easily reached. One point equals half an hour of screen time.
We also have this colored behavior chart hanging in the kitchen. Every morning their personal clip starts in the middle (white.) Good choices move you up. Bad choices move you down. If you reach the bottom, you're "busted" which means you lose some privilege or get extra chores. If you reach the top and are still there at bedtime, then you earn a bonus screen-time point.
Now that Quartus is 3 and a half, he is on the behavior chart and point system, too. Though, we are less strict with him about it still.