Umpire Field Clinic - A very solid, complete umpire field clinic from the Andover, MA Little League Chief Umpire. First 36 minutes around plate mechanics. Base mechanics from 36 minutes to about 65 minutes; Last 25 minutes around general precepts. 

Plate Mechanics - A very solid video focused on LL plate umpire mechanics and duties

A Youtube playlist of solid little league mechanics videos.