Thanks to Stephen Covey (of “7 habits” fame) for popularizing if not inventing this metaphor / concept handle, and to David Ernst (of Secure Internet Voting fame) for pointing that out. Also to Scott Alexander for the concept handle concept handle.
If you’re failing to carve out time for something that’s important but never time-critical, here’s a self-helpy analogy with rocks and pebbles in a jar. It’s simple and memorable. We refer to it often enough that we wanted something concise to point to, et voila.
So the jar is your life and the rocks and pebbles are things that fill your life — things you can spend time on. The pebbles are the little things like email and errands, often time-sensitive and thus seemingly urgent but not that important. 
The rocks are the opposite: things like spending time with family, playing music, learning new skills. If you’re self-employed then billable hours might belong in this category.
What makes the analogy memorable is to watch what happens when you let the pebbles vs the rocks take precedence. If the pebbles go in first then the whole jar ends up full of pebbles, with no room for any rocks.
But if you put the rocks in first, plenty of pebbles (the truly urgent ones) will still fit in around them.
The moral: Get those rocks in that jar!
 Traditionally this metaphor is taken further: rocks, pebbles, sand — even water. A jar full of rocks can still fit more pebbles. A jar full of rocks and pebbles can still fit sand. And… you get the idea. I’m not sure what the sand and water correspond to in terms of getting things done though. Eating and breathing?