I am not great at it but I think shoulder and arm energy and tension may detract rather than help, especially when you're getting started. This is what worked for me (though as I say I am not great at it at all). I think if you start with the bellows closed and move the bellows slowly in and out from the forearm and wrist with as little tension as possible, a small movement, as little as possible movement in the instrument overall, and sort of pulling back at the end so there is no collision on the way in - try something like that, although I have not described it very well. Then if you want to increase the volume, you are talking about the same thing but increasing the distance that your wrist moves through. I practiced scales shaking in and out on each note but I think a good way to practice it is playing a tune you know and building in a shake on a few notes only, that way you have to keep control. A good balance on the instrument and being well strapped in, and having the straps so that you can move your left arm freely and move the bellows in and out perpendicular not at an angle, all these things make it easier I think.
That's what I have learned/experienced anyway, or to be honest more what I've seen others do. I am learning one tune that is half shaken at the moment and getting to grips with it has involved putting in less and less energy, not more. But if you look at some great "shakers", they are not necessarily shaking the instrument hard.
