Try and think that there is a string pulling you up from your head
Put a crop through your arms at your elbows
Pretend like you were trying to sit on your backpack pockets in the saddle
shoulders back and hips under
My friends used to put a whip down her back and it really worked
Do some core excercises 3-5days a week. Yoga focusing on core strength is amazing. When it comes to lower back strength, it is all about your core strength.
Maybe try a brace to try and retrain you back to a straight riding position
Put a crop in between your elbows. It helps sort of straighten up and keeps everything in line.
Try to always think keep your shoulders back
Roll your shoulder's back, look forward, toes up heels down, chin up.
Put your feet and shoulders back, and keep your chin up.