This verse is so important. If we aren't aware of what we are doing we can open doors for the devil to mess with us.
What opens doors? Sin, unbelief (which is a sin too by the way), and then a host of other things that actually fall under one of those categories. Fear is a big one. When we fear we can't have faith.
We are in a battle. If we let up the enemy will slip in immediately. His minions are everywhere. The enemy hates God's kids because God loves us so much.
Don't make holes in your life.
Know this -- sin deceives. When we sin it messes with our minds. There is no little sin. If you know you have sinned, repent and turn back to God. Don't stand for compromise.
A word of caution: don't become the sin police for other people. We have no idea where they are at with God. We have no idea what God is doing in their lives. Leave the sin convicting to the Holy Spirit. You have one person you can deal with on the issue of sin -- yourself.
Be strong. Be tough. He who is within you is greater than everything in this world!