From a homily by St. Augustine:
You would seek to act justly? Well, “The just Man lives by faith.” Anoint the feet of Jesus; that is, follow in the Lord’s footsteps by living virtuously. Dry His feet with your hair. If you have surplus goods give them to the poor, and you will have dried the Lord’s feet with your hair; for hair would seem to be non-essential to the body. Here is something to do with your superfluous goods. What is superfluous for you is necessary for the Lord’s feet. Here on earth the Lord’s feet may be suffering want.
— Treatise 50 on John, near the beginning, as quoted in the Roman Breviary.
If you have no superfluous goods, give of your time. If you have no superfluous time, offer your prayers. If you have not even prayers to spare, offer a sigh and God will fill in the rest.