What goes on between our ears matters to God, probably a lot more than we realize. Equally important is the Bible's clear teaching that we are responsible for our "thought life" and called to a holy thought life, glorifying to God. Easy to say, hard to do!
How do I deal with wicked thoughts? How do I become more thoughtful?
The basic Scriptural pattern of "repent and replace" applies here as well as it does in other areas of life. (See Eph. 4:22-24, 28-29 for how Paul applies this principle to thieves and those with poor speech.) No Christian is automatically given a mature, godly and productive thought life at their conversion. It's a part of wisdom and maturity which must be gained by prayer and prayerful labor.
So whenever wicked (impure, lazy, selfish, etc.) thoughts pop up, the first step is always repentance. Jesus' teaching in the Sermon on the Mount revealing anger to be murder of the heart and lust to be adultery of the heart should lead us to sprint toward repentance every time the ugliness of sin sprouts in our inner self.
As part of repenting, you might also consider what it is that makes us less thoughtful. Surely the incessant noise of 21st century America is regularly pulling us away from thinking for ourselves. Too much "mindless" entertainment (which generally does our thinking for us) never sharpens our mind. Other sin we're holding onto is likely to damage our conscience, keeping us from being able to truly see, hear or think on truth (Acts 28:27).
The next step is just as important. An empty mind is of no value to you, and can be simply a blank canvas for more sinful thoughts (consider Mt. 12:44). We must take charge by embarking on a mission of replacement. I've often thought Philippians 4:8 is a great theme verse for a godly thought life: Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things. By the Spirit's help, we must put these types of things into active rotation in our thought life.
How? The same things Christians have been doing for millennia! Prayer, singing Psalms, learning and reciting Scripture, developing genuine fellowship with godly believers, reading material which draws your eyes to heavenly things, meditating on last Sunday's sermon...these are all activities of replacement. This is all ready and waiting for you – the key is you using your Christian will to do something with your mind that isn't always easy: think better.
For more fodder, consider godly examples of thoughtful people. You might consider Jacob, who studied the providence of God (Gen. 37:11), and Solomon, who prayed for an understanding mind (1 Kings 3:9), and Mary, who "treasured up all these things, pondering them in her heart" (Lk. 2:19). Sanctified by the Spirit and His Word, your mind will increasingly reflect that Scripture itself, granting you discernment between good and evil.
This is part of the Christian calling! Therefore, preparing your minds for action, and being sober-minded, set your hope fully on the grace that will be brought to you at the revelation of Jesus Christ. (1 Pet. 1:13) May God give us His Spirit's aid to train our minds for action.