Jesus is both pragmatic and utopian in Matt 6:19-21. In a day when wealth and riches meant oppression of the poor and a day when poverty, if handled properly, meant piety, Jesus summons people to follow him who will trans-evaluate possessions. Possessions then and now were and are a source of pleasure and a focus of our efforts. Think of the joy that many of us have when we get something new. As for us, we are quite glad that we now have a garage — we’ve lived here for twenty years without one and now have one.

It is easy to see what Jesus is saying here, but it is not easy to live with what he is saying. Simply put, Jesus says this:
1. Since the accumulation of possessions doesn’t bring us to things last that long (they erode and corrode, they can be destroyed and stolen), don’t focus on them. (Notice here, as Allison observes, the issue here is not possessions per se but the accumulation of possessions.
2. Since we are destined to live eternally, focus on what will be important then.
3. Our “treasures” (the accumulated things) indicate where our heart is.
Many are (fairly, I think) concerned when Christians focus far too much on “heaven” and not enough on “earth,” and I’m not so sure Jesus’ view of “kingdom” is not significantly earthy in its orientation. Jesus’ vision is not focused on heaven as we often think of it: it is focused on the kingdom. Still, even with that clarification, Jesus had in mind a future that would not be stained by death or sickness and the like. His vision is utopian. Jesus wants his followers to live for what matters most and for what will stand the test of time. In essence, he’s calling his followers through the rhetoric of kingdom to establish their priorities around his teachings.
The problem, as I said above, is not discerning what Jesus said; the problem is discerning how to live, in the nitty-itty-bitty-gritty of everyday life, in such a way that our priorities are focused on what is most important. The problem for most of us is not discerning the good from the bad (that is normally a routine decision) but the good from what is best.
What most Christians find here to be of most use is to learn to live on less in order to check desires, to discipline our spending, and to live below (rather than at) our budgets. What we need to focus on is not trusting in our savings or what we have that is stored away. More importantly, we need to live in such a way that our time-management is such that we have time to do what is most important. I find the “accumulative spirit” of Jesus’ words here most comparable to working extra hours or doing too much so that we can have more or do more and retire earlier. In other words, I can’t think of a passage in the Sermon on the Mount much more out of sync or counter-cultural to Western life than these three verses.
Some in the history of the Church have given everything away in order to follow this teaching of Jesus. Jesus is not one to give rules to make following him easy: it is a matter of discernment, of faith, and of disciplined practice that bring wisdom. Wisdom comes when we learn that the material is immaterial. What we strive for is freedom from possessions and the accumulative spirit.