My guess is your shake is your arm trying to balance the weight
Try pushups with various hand positions (outside, inside, etc) from different and/or unstable surfaces. In my various workouts I do around 400-500 pushups a week. I have and love my Bosu for pushups!
Alternatively just find something much heavier to hold out as practice.
This is not me however he has great examples. http://www.youtube.com/watch?v=FFsgOBbXlOU