I get this pain in my shoulder after a few hours of coding. It can last for weeks if I don't stay away from the computer. What do you guys do to prevent shoulder and neck pain?

for shoulder pain: i increase chair height, or get a lower desk where keyboard sits lower than my chest

neck pain: dont look at the screen upwards. have it lowered, too... well, as much as u can

definitely go OH &S crazy, check your desk / chair setup. Interrupt yourself and check your posture, are you tensing your shoulders, neck etc whilst working.

I've sometimes had to force myself to take a break every half hour and get up stretch exercise a little too prevent aches and pains.

could be worth seeing a physio too, they may be able to recommend some stretches to do :)

Back exercise.
Specifically anything that pulls your shoulders back (rows, deadlifts, pullups). Reaching forward for mouse and keyboard inevitably makes you round shouldered, need to counteract it.
And getting up and stretching regularly as was mentioned before

Thanks guys!