The biblical words for spirit (ruach in the Hebrew, pneuma in the Greek) both bear the basic meanings of "wind" and "breath." This shouldn't just inform our understanding of spirit; it should inform the implications of spiritual reality.
Like wind and breath, spirit is something invisible that has visible effects. We can't see the wind, but we can see leaves rustling. On an extremely gusty day, you can look up into the sky and see nothing extraordinary, but if you ran up a kite, the force of the lift would require some real strength to temper.
This is simplistic, I know. But the illustration makes a very serious point:
The Christian's Spiritual life is the invisible having visible effects. This is a tough reminder for all the cheap gracers with Jesus as their MySpace hero: If your spirituality doesn't have a visible effect, it's crap.