Egg shakers are great for preparing the left arm and hand muscles for vibrato! I learned this trick from my teacher-trainer Joanne Bath! It's as simple as holding the egg shaker in the left hand and shaking it and it is useful for practicing both arm and wrist vibrato. For a more refined and metered approach, you can practice with a metronome as the girl in this video is doing! One shake per beat, then two, then four, etc. https://vimeo.com/125569841
And as we artists know, presentation matters; that's why I'll be presenting the eggs from a lovely little basket and wearing a pair of bunny ears at group tonight. :)