Change something physical: location of your desk, desk organisation, team, job, wallpaper ... and a the same time stop playing mahjong, chess and read HN at work. Just stop completely, and never do it again.

Your brain will get accustomed to this new order of things, and will keep it as a given: when at work, I work.

Changing something physical in your work environment is _required_ for your brain not to fall back to its old habits.