Sri Chinmoy gave the following answer on a question about how to have more purity at Auckland (New Zealand) Airport on 9 July 1989 before returning to New York.
If you have any wrong thoughts, impure thoughts - no matter what is in your mind - look at a flower. As soon as you look at a flower, your mind becomes beautiful, your mind becomes pure, your mind becomes fragrant. If you look at a flower, your mind will be filled with pure thoughts. But you have to look at the flower most sincerely and intensely. You cannot gaze at it casually and allow your mind to roam freely. Your power of concentration has to be on the flower.
If you do not have a flower, you can also use a candle flame. Do not concentrate on the base or the wax, but only on the flame. Imagine that you have become the flame itself. By looking at the flame, you can get purity.
One way is to observe a small object, such as a flower or a flame. Again, by observing something larger than the largest, you can also develop purity. Look at the sky. That is the first step. The next step is to feel that you are inside the sky. First look at it, how vast it is. Then look at one portion of the sky and imagine that you have entered into it. Once you have entered into it, try to become the vastness itself. Impurity comes from the limited mind, whereas purity comes from vastness. The human mind is impure because the human mind is unable to become one with the Universal Reality. If something is vaster than the vastest - the sky or the sun or the Himalayas - then that vastness itself can give you purity.
Another way is to imagine the ocean. On the surface you are seeing that the ocean is full of turmoil and restlessness, but at the bottom there is poise. You can imagine that you are entering into the depths of the ocean.
These are several ways you can try. You can look at a flower or a flame. Again, you can choose something that is vaster than the vastest, higher than the highest or deeper than the deepest. Imagine that thing and try to enter into it.