There are a few things you can do. ridergirl, silverspur, and flytobecat covered the main ones.
It is entirely possible that you are still too tense, especially through your back. Also, that you are carrying too much weight in your stirrups. I would suggest trotting around bareback for a few minutes before you saddle up and concentrate on how your muscles are working and which ones you are using. When you get the saddle on, trot off and see if you notice a difference.
I haven't had this issue since I was a little kid, but I remember my mother telling me that having a saddle on the horse was not an excuse to get lazy. She had me try the exercise I outlined above, and I discovered that I wasn't using any muscle with a saddle on. I relied way too much on my stirrups.
Learning never stops