You could do a lunge lesson the next time you have a lesson with your dressage trainer. Whenever I feel that my position needs work, I ask my trainer for a seat lesson. I find it to be so helpful and I can really tell the difference when I ride on my own after that.
If you can only take a lesson every couple of week, make sure to get homework from your trainer so that you have stuff to work on between the lessons, exercises, figured and the like. I don't know if all trainers are like this, but I'm always free to call my trainer if I get stuck and need her to explain things to me.
Also ask your trainer if there are some dressage books that she reccomends and read up! It's cool when you can begin to apply something you've read about into your riding.
"He doth nothing but talk of his horses."