A bad habit is usually connected to wrong beliefs and attitudes. We should not want to change a habit only because it is embarrassing, expensive, unhealthy, or makes us feel guilty--rather we should want God's greater purpose for our life to be fulfilled. Until we deal with the underlying wrong beliefs that weaken our resistance to the bad habit, we will have only limited success in overcoming it.
The Christian must see that bad habits are ultimately spiritual issues. We must not hesitate to call them sins. And we need to realize that the means of sanctification described in Scripture (chiefly the Word of God and prayer) are essential for overcoming such habits.
We are responsible for our own sin--including those sins "which so easily beset us." The fact that we do something wrong habitually does not relieve us of responsibility. On the contrary, it may make the sin all the worse. So we must take personal responsibility for our own habits and not shrink from calling them sin.
Sinful habits are not insurmountable problems for the Christian. After all, the Holy Spirit indwells us and is working to conform us to the image of Christ. And if God be for us, who can be against us? (Romans 8:31) Furthermore, Galatians 5:16 says that if we "walk by the Spirit, [we] will not carry out the desire of the flesh." And 1 Corinthians 10:13 is a promise that God will not permit us to be tempted beyond our ability. If we make use of the resources provided by God through His Spirit and His Word, we can attack any habit knowing that we can win.
Let me make some practical suggestions for overcoming bad habits that I think you will find helpful.
First of all, remember that sin begins in the mind. James 1:14-15 compares a person falling into sin to a fish or animal that is caught with bait. It sees the bait, desires it, and is trapped in the process of grabbing it. Likewise, sins that ensnare us begin in the mind.
The person who steals first thinks about the thing he wants. He then thinks of a plan for getting it. After he has schemed in his mind, he takes it. He could have stopped the sin in his mind before it became completed in his action. That's why Scripture commands us to renew our minds (Romans 12:2), to think about good things (Philippians 4:8), and to meditate on the Word (Psalm 1:2).
Defeating a habit also requires changes in lifestyle. We are to make no provision for the flesh (Romans 13:14). The person who wants to stop smoking should throw away all his cigarettes and not buy any more; the person struggling with sexual sin should get rid of any suggestive materials in his possession. Avoid the company of those who have the same problem (1 Corinthians 15:33), and avoid the places and circumstances which tempt you.
Finally, don't try to battle a bad habit alone. Develop relationships with more mature Christians who will encourage and support you (Galatians 6:2). Spend time in prayer with them. Ask them to "check up on you" regularly, and be honest about your failures (James 5:16).
Biblical change is not just turning away from sin; it is turning toward righteousness. The person who lies must speak the truth (Ephesians 4:25). One who steals must not only stop stealing, he must work and give to others (verse 28); and one whose language is unwholesome must learn to edify others (verse 29). Do not just stop sinning--start doing what is right. The good habits you build will replace the sinful ones.
For further study about overcoming sin in your life, consider the sermon in the Also Available section.