From Chapter 1 of Walden, by Henry David Thoreau:
“A man who has at length found something to do will not need to get a new suit to do it in; for him the old will do, that has lain dusty in the garret for an indeterminate period. Old shoes will serve a hero longer than they have served his valet — if a hero ever has a valet — bare feet are older than shoes, and he can make them do. Only they who go to soires and legislative balls must have new coats, coats to change as often as the man changes in them. But if my jacket and trousers, my hat and shoes, are fit to worship God in, they will do; will they not? Who ever saw his old clothes — his old coat, actually worn out, resolved into its primitive elements, so that it was not a deed of charity to bestow it on some poor boy, by him perchance to be bestowed on some poorer still, or shall we say richer, who could do with less? I say, beware of all enterprises that require new clothes, and not rather a new wearer of clothes. If there is not a new man, how can the new clothes be made to fit? If you have any enterprise before you, try it in your old clothes. All men want, not something to do with, but something to do, or rather something to be. Perhaps we should never procure a new suit, however ragged or dirty the old, until we have so conducted, so enterprised or sailed in some way, that we feel like new men in the old, and that to retain it would be like keeping new wine in old bottles. Our moulting season, like that of the fowls, must be a crisis in our lives. The loon retires to solitary ponds to spend it. Thus also the snake casts its slough, and the caterpillar its wormy coat, by an internal industry and expansion; for clothes are but our outmost cuticle and mortal coil. Otherwise we shall be found sailing under false colors, and be inevitably cashiered at last by our own opinion, as well as that of mankind.”
P.S. Update, 2023: I still think about this quote with some frequency. Keeps me from taking up rock climbing, for example.