I once saw a nice bobbing training method on a movie, I think it was J-Lo on the lead but I forgot the title of that movie. In one of the scenes, she just put up a line, just like the old clothesline, probably a wire, where she would bob her head up and down, to the left and the right of the string while standing in a squat position. This could be a standard training method but that was the first time I'd seen it. A clever method I'd say.