My old coach used to say "pretend to be dolly parton".
Also, lean back until you feel like you are going to lay down on a couch. If you're having problems with being too forward, you wont actually look like you're leaning too far back, it will just feel like it (because it's further back than you are used to).
There's a product called shoulders back which helps riders to keep good posture.
Kara, it's hard for me to tell for sure from your profile picture, (old eyes), but it looks as though your heels are a bit up and a bit behind you. Try standing straight up in your stirrups, then lowering yourself down in the saddle keeping your toes in, dropping your heels. Imagine a straight line from your ear to shoulder to hip to ankle. If you have someone who can help you, have them literally hold a measuring stick up alongside you to check where you're off-balance.
I have the same problem. My trainer told me to keep my inside arm straight to the sky. I use only my outside rein to steer and find to keep my arm raised correctly I must have perfect posture. If I can see my hand I'm too far forward.
Hayley has great advice. A fun little trick is to imagine a string on the top of your head that is pulling you upwards. Also try to have someone on the ground tell you when you are leaning too far forward and when your body is straight. When your body is straight, you'll probably feel like you're leaning really far back, so it'll help to have someone there to reassure you.