How should one constantly contemplate upon God?

For this, Jagadguru Shree Kripaluji Maharaj discloses a secret.  He tells us to chant the divine name of God with every breathe that we take.  Our mind needs a base to which it can get attached.  Otherwise, how will you attach your mind?  So the solution is to firstly, chant his divine name.  When you chant his name, your mind will naturally think about God.  So, chanting of God's name is a very good means of absorbing your mind in God.  Besides chanting, you can also visualize and meditate upon the divine form of God.  The third point is to think and sing the glories of God's qualities.  He is the friend of humble souls and purifier of sinners; He is the ocean of mercy and Grace, more beautiful than infinite cupids.  He is the soul of our soul.  By contemplating upon these qualities of God, we should divert our mind towards him.

You should think about the leelas (pastimes) of God - He is sitting, we are fanning him while he rests, he is sitting on a swing, etc.  You can imagine any leela of God as per your wish and meditate upon it.  In addition to the leelas, think of his divine abodes.  If you are unable to concentrate upon God then you can think about Vrindavan.  Think that the tree nearby is a divine tree from Vrindavan.  From every leaf, the sound of chanting of 'Radhey Radhey' is emanating.  Or, you can meditate on the form of the Guru.  The main point we have to remember is that God has infinite Names, infinite Forms, infinite Attributes, infinite Pastimes, and infinite Associates, and that are all one.  They all reside within each other.  Therefore, God's divine Names, Forms, Pastimes, Abodes, Qualities, and Associates, all belong to the divine realm of God.  By meditating on any of these areas, your devotion is strengthened.  This is the foundation for practicing devotion.  If you attach your mind to any of these divine attributes, it is divine and you will get the divine result.