Seneca’s “On the Shortness of Life” is a must-read for anyone who feels unsure how to accomplish all the things in life that seem to need doing. A commonly quoted passage: “T]he time we are given is not brief, but we make it so. We do not lack time; on the contrary, there is so much of it that we waste an awful lot.” 
Here is a partial list of the things that Seneca feels are a waste of time: drinking, being lazy, being obsessed with one’s career, making money, pursuing a business deal, living in a rage and being obsessed with harming others, working for the wealthy, chasing other people’s money, complaining about one’s finances, arguing with one’s spouse, berating underlings, fulfilling “pointless” social obligations, discussing trivia, playing chess, “jabbering . . . about fantasy and nonsense” . . . the litany of time-wasters goes on and on.
I was struck by Seneca’s irascible tone in this work. There is little trace of the generosity of spirit that characterizes his letters to Lucilius. He seems less like someone you’d want to enjoy a glass of wine with, and more like someone after bidding adieu to whom you’d need a bottle of wine to calm your nerves.
And yet, amidst the railing against the folly of the way the vast majority of people spend their days and their lives, he shares some valuable guidance on things you must do to avoid wasting your life. He writes:
Those who choose to have no real purpose in life are ever rootless and dissatisfied, tossed by their aimlessness into ever-changing situations. A man who opts to live a life with no principles to steer by usually gets a big surprise from Fate while he is sitting back and yawning. 
Having purpose and principles is crucial, and we need to spend time to figure out what those might be for us. The time we spend on other things takes time away from this vital task.
Later in the essay he says that the only “really leisured people” are those who spend time on “acquiring true knowledge rather than trivia.” These people are
not content to live “in the moment” exclusively but show a keen awareness of history, of all the years that have gone before them and they know that the amount of time they have left is uncertain and finite.
At last, he gives us a prescription for our ills, specific suggestions for what we can pencil into our packed schedules: We should spend time with thinkers from the past, and in doing so, essentially lengthen our lives and “explore the vast ocean of time stretched before us”:
Because of the efforts of our ancestors, we have moved further from darkness into light. . . . When life permits us to commune with every era, why not turn the tables on this absurdly short and fleeting span of time we are endowed with by spending some of it in the past, which is boundless and inhabited by men better than ourselves? . . . It is fair to say that those who make Zeno, Pythagoras, Democritus and other giants of philosophy their daily companions will be more fully engaged in a rewarding life.
It is this streamlined approach to life that makes the Stoics so appealing. Preoccupied with cares? Just forget about the things that are out of your control. No time for philosophy? Just drop the things that don’t lead to discovering your purpose and having a rewarding life. “Steer away from the tempest of busy time and find a tranquil harbor to cruise into. . . . Most of your life is given to society, but save some time for yourself.” 
 Ch. 1;  Ch. 2;  Ch. 14;  Ch. 18.
Seneca translation by Damian Stevenson, found in the Stoic Six Pack 2 collection.