I suffer from this too from years of driving too much with work ! But am getting better now I've started working on it but have found the best time to practise is when watching tv or driving the car !
Here's a little trick you can use for anything !
Decide on a 'marker' i.e. a red dot (I buy the little round stickers from stationary stores you can get in all different colours).
Put yourself in the 'correct posture' and then look at the red dot, look away before you feel fatigued and then relax.
Repeat this several times (this varies per person but if done correctly 3-9 times is norm). This will create an anchor to the red dot so that whenever you see the red dot it will remind you to correct / check your posture. You can stick the red dot on your tv, on your car steering wheel and maybe you can put it somewhere like your glove when riding.
You may need to reanchor it now and again if you start switching off to the stimulus but you just then repeat the exercise.