How to fill mind with pure thoughts

The mind is filled with purity (Sattva) if you work without expectation of fruits, if you work for the sake of God, if you regard work as worship or Puja of Narayana, if you dedicate all your
actions to God as Isvararpana. Feel and think that you breathe, live and work for God alone every second of your life, and that, without Him, life is absolutely useless. Feel the pangs of separation
while at work if you forget Him even for a fraction of a second.