Time Management tips from "First Thing First"

I have been reading a lot of materials on time management these days and came across on this very useful tips from famous book from Steven Covey:
If you are having difficulty finishing all your work on time(in other words having your time management problem) consider thinking your work/activities as Rock, Gravel and Sand. Rock representing the most strategic and important tasks, gravel representing the next important tasks and sand the least important ones. Now you try to fill them in a bowl. If you fill sand and gravel first you will end up finding no place for rock. But if you fill the rock first, gravel second there will still be plenty of room for sand left.
Very powerful analogy!