The pay-yourself-first principle is useful because it helps you keep track of your money because it sends it straight into your account. This helps you know exactly how much you have, and makes budgeting easier, and it helps with saving money! You should teach your friend by explaining the principle, and then having him exercise it for a month. You should check up on your friend about every week, and then after they have maintained the routine for about a month, they should realize that it is helpful to them, and they will continue to use it from there on out.