Selections from an essay at The Atlantic:
Lesson 1: I will look upon death or upon a comedy with the same expression of countenance.Lesson 2: I will submit to labors, however great they may be, supporting the strength of my body by that of my mind.Lesson 3: I will despise riches when I have them as much as when I have them not; if they be elsewhere I will not be more gloomy; if they sparkle around me I will not be more lively than I should otherwise be: Whether Fortune comes or goes I will take no notice of her.Lesson 6: Whatever I may possess, I will neither hoard it greedily nor squander it recklessly.Lesson 9: I will be agreeable with my friends, gentle and mild to my foes: I will grant pardon before I am asked for it, and will meet the wishes of honorable men halfway.Lesson 11: Whenever either Nature demands my breath again, or reason bids me dismiss it, I will quit this life, calling all to witness that I have loved a good conscience, and good pursuits; that no one’s freedom, my own least of all, has been impaired through me.
The other lessons are at the link, each with salient commentary. In all fairness it seems appropriate to append this observation from the Wikipedia entry:
Even with the admiration of an earlier group of intellectual stalwarts, Seneca has never been without his detractors. In his own time, he was accused of hypocrisy or, at least, a less than "Stoic" lifestyle. While banished to Corsica, he wrote a plea for restoration rather incompatible with his advocacy of a simple life and the acceptance of fate. In his Apocolocyntosis he ridiculed the behaviors and policies of Claudius, and flattered Nero—such as proclaiming that Nero would live longer and be wiser than the legendary Nestor. The claims of Publius Suillius Rufus that Seneca acquired some "three hundred million sesterces" through Nero's favor are highly partisan, but they reflect the reality that Seneca was both powerful and wealthy. Robin Campbell, a translator of Seneca's letters, writes that the "stock criticism of Seneca right down the centuries [has been]...the apparent contrast between his philosophical teachings and his practice."