[muxxa]

None of the comments so far seem to have picked up on the mention of caffeine. For me this was the revelation; paying attention to how a strong cup of coffee alters my posture and increases tension from my back through to my wrists. I also notice myself long-pressing shift/ctrl/alt keys in a state which I can only describe as 'over-determination'. (Funnily enough, a large pot of green tea delivers a steadier dose of caffeine and doesn't seem to produce these negative effects)

Not everyone is affected this way and I believe the root cause that caffeine exacerbates is the particular kind of stress you feel when you want to get something done already, a typical state of mind to be in running a startup. Stress and psychological factors feed directly into bad posture and this is why taking regular breaks only upped my stress levels as now I needed to work even faster.

Lastly on the level of equipment fixes, ensure your elbows are supported by arm rests.