Like thieves in the night, unwelcome thoughts can and do seek entrance to our minds. But we don’t have to throw open the door, serve them tea and crumpets, and then tell them where the silverware is kept! (You shouldn’t be serving tea anyway.) Throw the rascals out! Replace lewd thoughts with hopeful images and joyful memories; picture the faces of those who love you and would be shattered if you let them down. More than one man has been saved from sin or stupidity by remembering the face of his mother, his wife, or his child waiting somewhere for him at home. Whatever thoughts you have, make sure they are welcome in your heart by invitation only.
We need to be sure that we don't let the adversary take control of our lives. The truth is we are behind the wheel-- and it's only when we let him take the driver's seat that he can take us where he wants us to be (which is a little town called "Misery"). It's amazing to fathom that-- why would we ever let him drive? Why would we give place to the enemy of our souls? I guess a lot of it comes down to the way sin is presented. The world is awash with messages of how appealing it is, how fun it is, even sometimes how right it is!
But none of that is the truth. I guess the important thing is to keep the Holy Ghost with us to help us know the truth (and remember it). It's a hard thing to "always remember". But we certainly can do it, with help from the Lord.