Keep your eyes fixed ahead and relax your seat and shoulders. I see a lot of riders constantly looking down (at the horses feet)
I have this problem a lot, so I lean "crazy far back" but it appears to be perfectly straight. When you think you're straight, have someone take a picture.
Breath and be more relaxed that is why we often leab forward. You may think your breathing but your not. I have been having the same problem and it is getting better. Good luck
Well you should firstly ask yourself why you lean back. Is it because you are tense while riding? Is it because you can't find your balance. Leaning forward is a symptom of the cause.
If you're tense try and have someone lunge you while you're on the horse and ride without your hands (as long as the person on the ground is in control), this also works for lack of balance. Another good but more rudimental way is to ride a little bit every day with your shoulders too far back so when you sit up again, you will be straighter.