Q: What is sanctification?
A: Sanctification is the work of God's free grace by which our whole person is made new in the image of God, and we are made more and more able to become dead to sin and alive to righteousness.

Ever notice how hard it is to break old habits? How difficult it can sometimes seem to do right, to be Christ-like? Our new self strives for this, while at the same time, our old self still fights for its own way. God's work of sanctification in our lives causes this new self to grow stronger & stronger.

23Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. 24He who calls you is faithful; he will surely do it. 1 Thessalonians 5:23-24 ESV

22to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, 23and to be renewed in the spirit of your minds, 24and to put on the new self, created after the likeness of God in true righteousness and holiness. Ephesians 4:22-24

We could work our hardest to be like Jesus, but if God didn't (by the indwelling of the Holy Spirit) sanctify us, it would be of no use. Just like justification, it is God's work in us. But that doesn't mean that we just kick back and don't do anything. If it were totally in God's hands, and we didn't have to do anything, the Bible wouldn't be filled with teachings and instructions on how to live a Godly life. We have to cooperate with the Holy Spirit in this process. The main thing is "dying" to ourselves - "putting to death" or resisting those sinful things we feel like doing. When we lead a self-centered existence, this is equated with idolatry:

5Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. Colossians 3:5 ESV

So, be mindful, be consistent and rememeber that

6And I am convinced and sure of this very thing, that He Who began a good work in you will continue until the day of Jesus Christ [right up to the time of His return], developing [that good work] and perfecting and bringing it to full completion in you. Ephesians 1:6 AMP