Mary Oliver

To live in this world

you must be able
to do three things:
to love what is mortal;
to hold it

against your bones knowing
your own life depends on it;
and, when the time comes to let it go,
to let it go.

– Mary Oliver

To read more of her poetry, read:
New and Selected Poems : Volume One by Mary Oliver
New and Selected Poems : Volume Two by Mary Oliver

Also:
Rules for the Dance : A Handbook for Writing and Reading Metrical Verse by Mary Oliver
A webpage with her poems
Academy of American Poets profile