If the mind is busily engaged in conceptual activity its hard to practise mindfulness. Nevertheless, you never stop breathing, and your body is always in one posture or another.
Try to maintain mindfulness of your sitting posture, and all major movements of the limbs, getting up from your seat, reaching for the phone, etc.
Just learning to sit up straight while working will improve your breathing and alertness, while reducing the risk of back injuries, etc.
The medical advice is that sitting for long periods is very harmful — try to get up at least once an hour, at least to stretch your limbs.
• • • • (Upasampadā: 24th June, 1979)