Protip: in your car, adjust your rear view mirror, so its angled higher than your resting position. This way you're reminded to straighten your back and sit upright. Furthermore, the back of your head should touch the headrest.
(From previous replies to this advice, some of you have shit car-seats and/or head rests, so modify this advice to suit you.)
Protip 2: an amazing exercise to practice: stand with your back against a wall, with your feet one foot away from the wall. Try to press the lowest point of your skull against the wall, as high as you can (your chin should feel like it's tucked slightly).
Then, raise your arms flat against the wall, like the picture below. You'll need to move your arms around slightly, but you'll eventually feel the perfect spot where your spine begins to straighten out.
Hold this position for a minute. Do this multiple times a day and your posture will improve drastically.
My Mum has started doing flying fox (big bat) rescues this year. They are the most adorable creatures. To keep the pups feeling safe and secure, they're rolled up in a burrito blanket, and given a dummy. They're very sweet animals once you get to know them.
(Don't handle them yourself though, if you can avoid it, they typically have viruses which are deadly)