In a Troop I used to be affiliated with, before I left and started a new Troop, we used attendance as a carrot. At the weekly meetings a boy in uniform got 2 points, a boy in attendance but not in uniform got 1 point, and a scout not there got 0. At the end of the year, the boy with the most points earned 1/2 his summer camp fee as an award.
You could use other awards, but keep the attendance over a year and watch the uniform wearing increase...