The second reading for Mass today (click on the box in the sidebar) contains the following exhortation from St. Paul:
Brothers and sisters: Rejoice always. Pray without ceasing. In all circumstances give thanks, for this is the will of God for you in Christ Jesus.
I don't know about you; but of all Christian norms, I have found the above to be the absolute hardest to follow. I do not, and feel I cannot, "rejoice always." Much of what I must deal with, including some things about myself, doesn't seem worth rejoicing about. I do not, and feel I cannot, "pray without ceasing," even when I make an offering to the Lord of all that I do and undergo. And I most certainly do not give thanks "in all circumstances." I do thank God for dying to save us from the grip of Satan; I have no trouble doing so; indeed it would be crazy not to. I even thank him for letting me share, through baptism, in the Son's death so that I can, conceivably, rise with him in glory. But I have a very hard time being grateful to God for life just as it is. (Hence one of my favorite toasts is "to life as I wish it were.") When I'm feeling particularly frustrated or desperate, I am more inclined to criticize God than to thank him. At any rate, the attitude of gratitude is not easy for me to sustain. I suspect the same is true of many devout Christians who, understandably, would rather not admit it.
That, of course, is precisely why Paul's advice is so important. What he's doing, as much as exhorting, is describing life as filled with the Holy Spirit. For it is only in and by the Spirit that his advice can be followed. In a sense, I suspect, it is the Holy Spirit in us who does such things through us. All we need to be is willing instruments.
In my experience, the key to becoming a willing instrument is to strive always for objectivity. One's own little place in the divine tapestry of life is not all that clear from one's own point of view. Often, it is quite hidden. What matters to us as individuals, though it often overlaps with what matters period and thus matters to God, necessarily differs from it too. God has given us a general sense of what life is about, but how our own lives fit in is partially and necessarily mysterious. So, I can thank God for how my life fits in from his point of view, even though I am not vouchsafed that viewpoint and get only fleeting shadows of what he sees when I see at all. I can pray ceaselessly by striving to make my life revolve around him, his Mystical Body the Church, and her members—not around myself. I can rejoice always by remembering that, even though I'm just a soldier in the trenches who sees mostly mud, darkness, and the occasional shelling, I obey a General whose victory is assured and will bring me far better than I can even imagine.