This year, I've been mastering the fine art of bribery. I work with a very ADHD kindergartener, and pretzels motivate him like nothing else. For each letter that he spells right in a word, he gets one pretzel. He's up to twenty words per session now---he'll even spell pretzel and robot.
This is the little boy who was so quick to spell "shut up" when I taught him the "sh" blend. He's quick as lightning when he feels like working! Knowing his propensity for less socially accepted words, I waited with bated breath when I taught him about "ch." How many swear words would he come up with? Luckily, a plane flew by and he was up and at the window before the full impact of his newly expanded vocabulary was upon him.