Conquer Impure Thoughts.
When you are very busy in your daily work, you may not harbour any impure thought; but when you take rest and leave the mind blank, the impure thoughts will try to enter insidiously.
You must be careful when the mind is relaxed.
Thoughts gain strength by repetition. If you entertain an impure thought or good thought once, this impure thought or good thought has a tendency to recur again.
Thoughts crowd together just as the birds of the same feather flock together. So also, if you entertain one impure thought all sorts of impure thoughts join together and attack you. If you entertain any good thought, all good thoughts join together to help you.
~ Sri Swami Sivananda, Thought Power