How can a writer improve her craft?

Dear Literary Ladies,
What advice would you give a writer wanting to improve her craft? I read so many books on writing, and every one of them offers different techniques. Also, how long can I expect to work at this until I see results?

Each person’s method is no rule for another. Each must work in [her] own way, and the only drill needed is to keep writing and profit from criticism. Mind grammar, spelling, and punctuation, and use short words, and express as briefly as you can your meaning. Young people use too many adjectives and try to “write fine.” The strongest, simplest words are best, and no foreign ones if it can be helped . . .

Read the best books, and they will improve your style. See and hear good speakers and wise people, and learn of them. Work for twenty years, and then you may some day find that you have a style and place of your own, and you can command good pay for the same things no one would take when you were unknown. . . I have so many letters like your own that I can say no more, but wish you success, and give you for a motto Michael Angelo’s wise words: “Genius is infinite patience.”

—Louisa May Alcott (from a letter, 1878)