Completely the wrong approach. Fighting chronically tense muscles with more tension is not going to help anything, standing or sitting.
It worth noting that muscles can only pull. He should have been trying to relax the tense muscles rather than trying to counter with more tense muscles. This is not easy though. Have a look into the Alexander Technique or Feldenkrais Method, or the book Somatics by Thomas Hanna for some different approaches to re-educate your sense of kinesthesia.
These are popular with dancers, who generally move with a lot more grace and less effort than the average computer geek.
Far better is to reeducate yourself how to release the forward pulling muscles. This however is no easy task. Thommas Hanna has a good book about this with very interesting exercises to practice.
The Alexander Technique is another discipline that teaches you how to re-educate your mind / body control.
Watch professional dancers, how they appear to be both straight and relaxed and fluid with their movements. Not stiff and rigid. Also people who practice Yoga are both flexible, yet have better than average posture.
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