First, I start by playing a one-octave arpeggio (eighth notes). I start slowly and as soon as I hear that I'm playing evenly, then I branch out to two octaves (eighth notes), then three (eighth note triplets) and then finally four (sixteenth notes). Sometimes, I have to repeat a particular stage three or more times before I hear that I'm playing evenly.
For an added challenge, I change the articulation: staccato, legato, portato or slurred. Then, for a bonus, each hand will play a different articulation.
Beginner piano students can just stick with one-octave arpeggios. Junior intermediate students can branch out to two-octaves. Upper intermediate students can go up to three octaves, while advanced students should go up to four octaves.
Here's a video demonstration: