I recently listened to a talk by Joseph Goldstein entitled "The Buddha's Discourse on Non-Self" (no link, unfortunately, but you can find it on Dharma Seed). Naturally, I've already forgotten the name of the discourse, but in it the Buddha has a discussion with six monks about mindfulness of death. The upshot is that it is not enough to be mindful that we could die tomorrow or even at the end of the meal we are currently eating; we should consider that we may die after this breath or this bite of food.
This way of thinking would probably strike many people as rather morbid, and passages such as this may contribute to the common impression that Buddhism is about withdrawal from life. My experimentation with this perspective over the last couple of days has had precisely the opposite effect: the awareness of death has not only brought me immediately into the present moment, it has also changed the quality of presence. As the precious nature of the present moment is realized, preoccupations such as self-criticism or anger toward others become unaffordable luxuries. If all fear is really fear of death, as some have claimed, then fear itself becomes almost beside the point.
Though actual brushes with death can have a significant impact on us, the linkage of awareness of mortality and intimacy with life is such a commonplace in various forms of art and spiritual teaching that it has become something of a cliché. Aside from briefly snapping me out of the trance of life, I haven't found an awareness of mortality to be especially salutary, except as an excuse to avoid unpleasant tasks. Yet somehow, consideration of the possibility of death in the next moment, rather than the next day or even the next hour, seems to bring the mind more fully into the present moment. An hour or a day means time that needs to be planned.