Ideally, experts say, you should stash 10% of your income during your 20s. As you age, that percentage can grow—to 15% to 20% in your 30s. Use this to establish a rainy day fund. Again, in a perfect situation, you'd have enough to pay rent, food and other essential bills for three to six months. Those in more volatile industries—a start-up employee, for example—are encouraged to tuck away more.