When it comes to purifying thoughts, another aspect is the company we keep, who is around us. There are three points to this: if possible, surround yourself with people that have positive or wholesome thoughts. If you can, a sage or just anyone who is more pure or of more consciousness, so this will have an effect on your thoughts. And then finally, spend time in nature.
There once was a man, who was at a gathering with a sage and he had a burning question and with this he went to him. The sage said “Come back tomorrow.” He had waited a long time to ask him until the end of the gathering, but the sage asked him to come back the following day. The next day the man came back and this time he waited until everyone had left. Then he went up to the sage, who was inside of his cave. When he asked, the swami answered, “Wait, let me come out.” So he took him near to the bamboo trees, and he stood still, doing nothing. The man who came to ask the question was waiting for the swami to open his eyes so they could talk and he could get answers to his questions. Twenty minutes passed and when he still got no answer he asked, “Why are we here? Why can we not talk at the cave? Are you going to talk or are we going to stay standing like this?” The sage said, “If you would have gotten the point of this, you would have also gotten the answer to your question. The cave, its entrance and certain arts inside have been made by man, who has gathered impurities. Once we are with the bamboo trees, we become like that. Once we start looking at the moon, the stars, then we become like that.” The more we are in nature, the more purification happens in us and our consciousness.
In the Vishnu Pooran it says that just by living in the caves one attains to Vishnuhood, which means to be in a natural state. Nothing else, but just by living in the caves. But these days the whole area of Himalay is full of caves and full of people. They are not transformed. My understanding is that at the time the Vishnu Pooran had been written, there were less people in the caves, nature was more untouched. So the purification and reaching one’s natural state could happen to people. Their food was more pure, there was no pollution. But we can still learn from this, if we find places in nature where there is no involvement of societies and human distortions, this will enhance our purity and thus help reaching the natural state.