Everything I post has the word balance in it somewhere. Ensuring you are ‘in balance’ with the horse whilst he is constantly changing centre of gravity takes time to learn.
The trick is relaxation of the lower body, which includes those buttocks as mentioned in an earlier post. The upper body remains upright, supporting itself.
You should strive for the lower body to be completely independent of the upper body. Feel like you could unscrew yourself from the waist and separate the two parts. Aids with the legs should absolutely not affect the upper body.
You can achieve this by noticing whether you are collapsing in the upper body with the effort of the leg aids, if this is the case, re-establish your position and try again until your leg is able to go ‘on’ and ‘off’ independently of the rest of your body. Once again, you could be putting in too much effort!
It will take much practise, but just being aware of when you have achieved it and more importantly, when it is all going wrong will help you get there.