Stand up straight. If I heard that once I heard it 1000 times! It was said with the best and most loving of intentions but it was physically impossible without retraining my body.
Your tip for today is to train your body from the inside out. Sitting on the front edge of a firm chair, place your feet in mountain pose. Allow the arms to hang by your sides or rest your hands, palms up, on your lap. Then close your eyes and take a deep breath in. What happened? If you weren’t aware of anything occuring it is time to fine tune. As you take your next conscious breath allow the top of the sternum (breast bone) lift. Try not to use your shoulders. Then while you enjoy this space contract your belly to exhale and feel the low back and pelvis change ever so slightly. Continue to play with this and see what happens. Pretty soon breathing for spinal extension becomes a habit and you will sit and stand up straight with comfort and confidence.
Keep practicing and let me know how you are doing.