Ok, I lied - there is not one weird trick. There are THREE! And they’re SUPER EASY and NOT PARTICULARLY WEIRD (I DON'T THINK)!
Hours and hours of sitting causes the upper back to 'slouch' forward which can lead to restricted movement, pain and eventually a stooped posture (imagine a gorilla or a cartoon Igor or a cartoon Igor gorilla). But aside from getting to the chiropractor regularly, what else can you do to stop (or even reverse) this process?
Sit Less!
Yes, this one is not exactly rocket surgery but research shows that sitting all day can lead to all the aforementioned nastiness. So try stand up and move around a bit every 30mins or so - set an alarm, write yourself a post-it note… whatever it takes to remind yourself. Don’t wait until you’re already sore!
Exercise More!
Getting regular exercise (i.e. >3 days a week) has so many health benefits and this is yet another - keeping your body in a decent state of fitness will improve your mobility and tone your postural muscles. This means better posture and less pain. Which is a good thing, right? Right.

Start and Finish
Mobilise!
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Stand up from your chair and let your arms hang at your sides (do it NOW)
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Lightly tense your stomach muscles and then your bum muscles
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Pull your arms back and turn your thumbs out like a hitchhiker
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Try to lift your chest up to the sky/ceiling and slide your head back to make an awesome double-chin (see above for more)
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Pull your shoulder blades together and towards the floor
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Take a deep breath... And then relax back to the start position.
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Repeat for 3-5 times.
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Repeat this a few times a day.
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Thank me for preventing the aforementioned "Igor" scenario.