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 rear view mirror fell off awhile ago, thanks for reminding me I need to find some adhesive. Just seems counterintuitive to re-adhear it until after I get the growing crack out of the windshield. 18 wheeler threw a rock at me going down the interstate. Ugh.
Need to put some money away to replace it, insurance didn't want to help for less than the deductible.
The rear view mirror thing is a good tip. You've made me realise I sort of did this somewhat without being aware of it (I tended to be attentive when adjusting my mirror, so more often than not, it would be set in accord with good posture). Now I am aware of this, I can be much more deliberate in making this happen.