Donating when you have at least 25 florins and 10 pfennings (6010 pfennings) to a "well-known church" in towns and a religion stat above 30-60-90 will increase your virtue stat by 1-2-3 (religion/30) per character. The donation is 10% of the money you have and increases the virtue as long as you have more than 6010 pfs and a religion stat of at least 30, so the most economical practice would be to donate when you have that much - although it is a bit slower than donating two or three times between visits to the market.