Here, "Think" translates the Greek "φρονειτε," give or take an accent, which I'm told is an inflected form of the verb phroneo, meaning:
1. to have understanding, be wise 2. to feel, to think. a) to have an opinion of one's self, think of one's self, to be modest, not let one's opinion (though just) of himself exceed the bounds of modesty; b) to think or judge what one's opinion is; c) to be of the same mind i.e. agreed together, cherish the same views, be harmonious 3. to direct one's mind to a thing, to seek, to strive for. a) to seek one's interest or advantage; b) to be of one's party, side with him (in public affairs).
This kind of thinking, then, isn't an act so much as a habit or disposition. It's not a one-time use of your intellect, like thinking of a number between one and ten, but a way of using your intellect that St. Paul expects you who were raised with Christ to adopt.
To direct your mind to what is above is contrasted with the cast of mind mentioned in the first reading from Ecclesiastes:
For what profit comes to a man from all the toil and anxiety of heart with which he has labored under the sun? All his days sorrow and grief are his occupation; even at night his mind is not at rest.
A man's mind, says Qoheleth, is never at rest when it is directed to what is on earth. Jesus confirms that this is vanity with the parable of the rich fool:
"Thus will it be for all who store up treasure for themselves but are not rich in what matters to God."
But to think of what is above does not mean to ignore what is on earth. It matters to God that those who take His Name care for the widow, the orphan, and the alien.
Nor does it mean that Christians are obliged to ignore their own earthly needs or to pretend they have no earthly wants. But our earthly needs and wants look much different when our minds are striving for what is above.
I point this out because the bare statement, "Think of what is above, not of what is on earth," sounds, not only impossible (think of not thinking of an elephant), but impractical. We are, perhaps, too ready to ignore anything from Scripture that smacks of impracticality, but as is often the case this verse, when properly understood, is extremely practical. It tells you what to do, and not do, with literally everything.
By the way, here are all the verses in the New Testament that use "φρονεω":
Mt 16:23: He turned and said to Peter, "Get behind me, Satan! You are an obstacle to me. You are thinking not as God does, but as human beings do." Mk 8:33: At this he turned around and, looking at his disciples, rebuked Peter and said, "Get behind me, Satan. You are thinking not as God does, but as human beings do." Acts 28:22: "But we should like to hear you present your views, for we know that this sect is denounced everywhere." Rom 8:5: For those who live according to the flesh are concerned with the things of the flesh, but those who live according to the spirit with the things of the spirit. Rom 11:20: That is so. They were broken off because of unbelief, but you are there because of faith. So do not become haughty [i.e., "υψηλα φρονει," "think highly [of yourself]"], but stand in awe. Rom 12:3: For by the grace given to me I tell everyone among you not to think of himself more highly than one ought to think, but to think soberly, each according to the measure of faith that God has apportioned. Rom 12:16: Have the same regard for one another; do not be haughty but associate with the lowly; do not be wise in your own estimation. Rom 14:6: Whoever observes the day, observes it for the Lord. Also whoever eats, eats for the Lord, since he gives thanks to God; while whoever abstains, abstains for the Lord and gives thanks to God. Rom 15:5: May the God of endurance and encouragement grant you to think in harmony with one another, in keeping with Christ Jesus.... 1 Cor 13:11: When I was a child, I used to talk as a child, think as a child, reason as a child; when I became a man, I put aside childish things. 2 Cor 13:11: Finally, brothers, rejoice. Mend your ways, encourage one another, agree with one another, live in peace, and the God of love and peace will be with you. Gal 5:10: I am confident of you in the Lord that you will not take a different view, and that the one who is troubling you will bear the condemnation, whoever he may be. Phi 1:7: It is right that I should think this way about all of you, because I hold you in my heart, you who are all partners with me in grace, both in my imprisonment and in the defense and confirmation of the gospel. Phi 2:2: ...complete my joy by being of the same mind, with the same love, united in heart, thinking one thing. Phi 2:5: Have among yourselves the same attitude that is also yours in Christ Jesus.... Phi 3:15: Let us, then, who are "perfectly mature" adopt this attitude. And if you have a different attitude, this too God will reveal to you. Phi 3:19: Their end is destruction. Their God is their stomach; their glory is in their "shame." Their minds are occupied with earthly things. Phi 4:2: I urge Euodia and I urge Syntyche to come to a mutual understanding in the Lord. Phi 4:10: I rejoice greatly in the Lord that now at last you revived your concern for me. You were, of course, concerned about me but lacked an opportunity. Col 3:2: Think of what is above, not of what is on earth.
To say, "Think of what is above," then, connotes having a perspective according to, being concerned with, holding in regard, observing or following, being in agreement with, having the mind or attitude of, being occupied with, understanding what is above.