Through the death and resurrection of Jesus Christ and union by faith into that death and resurrection, you have been justified and definitively sanctified. The condemnation due to sin has been removed. You are a saint.
And this means that your relationship to sin has changed. When once sin held sway over you, you now have the power, through the Holy Spirit, “to not let sin reign in your mortal body” (Rom 6:12). Sin no longer reigns; righteousness reigns.
Unfortunately, however, sometimes sin feels inescapable. Sin feels so good. And you wonder many times why the power to say no isn’t there.
This is where an analogy helps.
When you became a Christian, sin was given a life sentence in prison. 
Sin has not completely died. It is still active in you. But in the prison cell, sin will not escape. Sin can ultimately harm you no longer. And over the course of time, sin is slowly dying in that cell. But until it is finally dead, it will do everything to justify its existence and to pull you into the prison cell with it to choke out your life.
What do you do, then, when it feels like sin is escaping the prison cell? The answer lies in Rom 6:11: we are to consider or believe sin to be dead—that is, in prison.
Considering sin to be in prison is an act of faith. You must believe that your relationship to sin has changed. To visit sin takes much more energy and planning than it used to. And sin can no longer hurt you like it used to. Its essential power has been broken, being tamed by the bars of prison.
Believing that you now have a new relationship with sin is the start to victory over sin. Why? Knowing that sin has been sentenced to life in prison, never to escape, but to rot in jail, will give you great confidence, power, and freedom to keep fighting what remains inside of you. The more and more you believe sin to be in prison, the more and more you will experience confidence, power, and freedom to fight what has been given a life sentence.
So, believer, whatever sins you are currently facing, believe with all your heart that sin is in prison.
Pastor Dan
For more on sin and definitive sanctification, see the education hour teaching found here: “Doctrine for Life – Godliness and Sanctification”