When I am without a computer, I practice the art of mindfulness. Mindfulness is indeed a habit that helps me focus and improve concentration levels.
As David Rock mentions in his book Your Brain at Work, Mindfulness is the director of the brain and helps us maintain our focus.
Research has proven that being mindful has lot of benefits, the primary of which is that it helps to relieve unnecessary stress by being present.
Read this wonderful insight on mindfulness, inspired from Your Brain at Work.