Adding acceleration or load
Once you have a good ‘feel’ for the basic out of saddle action described above, you have several options of how to progress. If you are looking for more strength, then finding steeper hills to work your repeats is the best way to go forwards. As the gradient increases, maintaining good technique becomes more difficult, but conscious controlled practice will give you the best results.
Adding more speed is your second option, which can either be achieved on a slight incline, or you can shift to flat sprints. To improve your speed up hill, you can use the same moderate incline and progressively accelerate up it so that towards the top your cadence is significantly faster than at the bottom. Switching your weight from left to right at a faster cadence out of the saddle can take practice too, so building into it gradually will help you stay in control.