The Kobriner Rebbe zy”a would say that a person sometimes accidentally sees something forbidden, or a person may have an improper thought fall into head. The way to avoid this is to work on purifying one’s thoughts so that when such things come up in his mind, he pushes them away immediately.
The Medrash (Bereishis Rabbah 44) says that the word “after” is used in the pasuk to mean that one should avoid the thoughts that come after he realizes what is occurring. One is commanded not to allow such thoughts to continue after he realizes that they are assailing him by quickly vanquishing them from his mind.