My lower leg is fine in walk, sitting trot, canter and jumps, however when I do rising trot it tends to jiggle about abit

In the words of my friend "you have jelly legs in riding trot"

I usually do sitting trot when doing lateral or anything slightly technical, but just wondering what I can do to make my lower leg abit more stable in rising trot? I'm going to be doing alot more no stirrup work and maybe some bareback but just wondering if there's anything else?
I want to start using the impulse spurs for lateral etc so I need to sort this out. The spurs will only be used in rising trot when my friend who is an instructor, and myself are positive I have the stability in my lower leg to use them
thanks!!