I'm not sure if they have corsets for guys,
but what helped me was to wear a corset (or bustier, whatever they're called in lingerie stories)
It helps you 'cause sitting/standing in any other position than upright just feels weird (as the boning gently pokes you in the ribs etc. as a reminder).
Another thing I learned in drama class was to pretend that you were a puppet and there's strings attached all over your body.
Right in the middle of your head there's also a string and if you imagine that being pulled up (position your body like it), it helps to keep in mind to stand up straight.
With dance class it was basically the old 'shoulders back, head up' thing. Pulling your shoulders back does help to keep your back straight though. And it feels very nice, the stretching of the muscles you can feel.
I tend to stand all hunched up as well, and what I do is exaggerate the shoulders back thing, as i'm not used to standing straight, but exaggerating it gets me in just the right position.
Another nice stretching exercise from dance class was this:
Stand with your feet slightly apart (about shoulder width).
Lift your arms up so your hands so they are slightly stretched out and are above your head, palms of your hands facing each other.
Now pretend your picking apples from a tree. But the apples are verrrrrry high up so you strrrrretch with one arm to pick the apple... you bring your arm down and try it with the other arm...bring that one down and repeat with the other arm etc.
It really helps to stretch your back and because you really have to reach (for the imaginary apples) it keeps your back straight.
Actually, if you're okay with following a dance class (ballet in particular), I highly suggest you do it 'cause it's one of the best things you can do for your back.