The following is a passage from C.S. Lewis' fantastic book "Mere Christianity". Every time I read through this work this passage strikes me. It was starting on page 160 and begins the chapter on "Hope."
Hope is one of the theological virtues. This means that a continual looking forward to the eternal world is not (as some modern people think) a form of escapism or wishful thinking, but one of the things a Christian is meant to do. It does not mean that we are to leave the present world as it is. If you read history you will find that the Christians who did most for the present world were just those who thought most of the next. The Apostles themselves, who set on foot the conversion of the Roman Empire, the great men who built up the Middle Ages, the English Evangelicals who abolished the Slave Trade, all left their mark on Earth, precisely because their minds were occupied with Heaven. It is since Christians have largely ceased to think of the other world that they have become so ineffective in this. Aim at Heaven and you will get earth 'thrown in': aim at earth and you will get neither. It seems a strange rule, but something like it can be seen at work in other matters. Health is a great blessing, but the moment you make health one of your main, direct objects you start becoming a crank and imagining there is something wrong with you. You are only likely to get health provided you want other things more -- food, games, work, fun, open air. In the same way, we shall never save civilisation as long as civilisation is our main object. We must learn to want something else even more.
I understand Lewis to be looking at life much as St. Paul did as he wrote to the Philippian Christians "for to me, to live is Christ and to die is gain. (Phil. 1:21)" It is common and even appropriate for Christian to yearn for the days of sight in heaven. We can take great comfort day after day knowing that that gift (eternal life) is a present reality for us. Through baptism and faith we are owners already of it. Yet, we don't yearn for heaven to the detriment of our joy in our days on this earth. Rather, the joyful yearning for heaven is to add more to our days on earth. Or as Lewis put it, "aim at heaven and you will get earth 'thrown in.'"