Justin Taylor posted this last week and I found it helpful, so I had to pass it along.  John Piper, as you may know, has some poetic sensibilities.  So when he preached on sustaining grace from Jeremiah 32:36-41, he wrote a four-line stanza to define sustaining grace.  Read it slowly at least twice and consider its truths.  You may even want to commit it to memory for when you encounter your own trouble, pain and darkness.

Not grace to bar what is not bliss,
Nor flight from all distress, but this:
The grace that orders our trouble and pain,
And then, in the darkness, is there to sustain.