BREATHE! We tend to forget to breathe while we're riding. Getting into a steady rhythm with your horse will also help you get in tune with him.
Also, the best and fastest way to learn to trot is to drop your stirrups. Posting without stirrups is painful and frustrating, but is the best exercise to develop balance, rhythm and strength. Also, get a grab strap to attach to the front of your saddle so that you don't rely on the horses mouth. It will also help you learn how to open and close your elbows as you post to get your hands softer.
Some other exercises you can do:
1. Post with one stirrup. Switch sides every so often.
2. Stand in your stirrups. Try standing as straight as you can for as many strides as possible. Use your grab strap (or horses' mane) if you lose your balance!
3. Vary your posting. Post for 5 strides, sit for 3 strides, post for 5 strides, stand for 3 strides. Changing it up will help you get balanced quick!
Hope this helps!