We must train ourselves, as in the story of an Indian brahmin who began making two piles of pebbles: he placed a white pebble in one pile for every positive thought he had, such as faith and devotion, and a black pebble in the other pile for every negative thought. At first most of his pebbles were black and the white pebbles were few. Great regret overcame him when he realised that he had such a huge amount of negative thoughts and so few positive ones. But as he persevered — by developing antidotes, adopting positive acts, and rejecting negative ones — the amount of white pebbles gradually increased, until the piles of white and black pebbles were equal. Seeing that there were still many black ones and fearing that negative thoughts would cast him back into samsara, he trained his mind again and again, until at last there were only white pebbles. 

-- Dilgo Khyentse Rinpoche