I've had bad posture for a long time, and I've tried to improve it over the years. Your posture should kind of go in a little at back of your neck, in a little at the lower part of your back, and out in the upper part of your back. It's not supposed to be perfectly straight. What seems to help most is strengthening your back muscles. Doing arm/shoulder exercises helps your upper back. I made up my own arm exercises which kind of helps-I just pretend I'm doing various swimming strokes. Supposedly stomach muscles help strengthen your back too, and neck rolls are good. Or try clasping your hands with your arms behind your back, and lift your arms away from you. Or, bring your arms out to the side in a big U, then bring them together in front of you, touching from the hands to the elbows. I think a portion of bad posture can come from low self esteem or feeling self conscious. Notice when your posture is especially bad. It might feel weird to stand up straight, but when you develop more muscles, it feels more normal and not like you look like you're trying to stand up straight.