Try again and again. Evolve feelings of intense yearning for the Divine vision of God. When the heart and mind will melt in the remembrance of God, tears will be shed; then the impurity of the mind will dissipate and then God will forever rule over our heart.
Upon experiencing tears, be careful to not let pride sneak into your mind, else you will also lose them. All tears are useless until unity with Shyama Shyam. When real tears will come, then God Himself will have to come and wipe them.