When Does a Passing Thought Become Sin?

Here is something I've been thinking about.  Every day we have thoughts that pass through our minds.  Sometimes they are not good thoughts.  Let's say for example you are at the grocery store check out and there are magazines with not so clean headlines or photos on them right in front of you.  You couldn't avoid them. Now a thought enters your head.  Have you committed sin because this thought entered your head as a result of something you didn't mean to see but couldn't avoid?

You are a guy and you want to go swimming with your friends at the pool.  But your eyes are assaulted with women wearing skimpy attire.  It causes thoughts to arise in your mind. Have you committed sin because the thought came into your head?

Is it possible that a man or woman can train themselves at the first thought to avert their attention to something else  and "put that thought to death" and thereby not have sinned?

Is it possible for a man to go to the swimming pool and ignore what he sees and not sin in thought?  How can he manage this?

I've heard it said that you can't always control what thoughts may pop into your head but you can control what you do about them.  A thought that pops into your head is not sin unless you entertain it, dwell on it, fantasize about it and/or act upon it.  If the minute it pops into your head you "put it to death" by turning your attentions to something more wholesome then you have not sinned in your heart.  Is this true?  What do you think?  Can you give Biblical references that speak to this?

What does the Bible have to say about these matters?