There is a lot of evidence to show that if an individual agrees to do something for you once, there is a significantly larger chance they will react positively to a larger second ask. This is because people like to be consistent in their actions. In an experiment in america households were asked if a large publicity banner could be put in their property, only 17% agreed, but when asked first if a small banner could be erected and then later asking to put a large banner the number of positive responses rose to 76%.

For donations this shows that a) it is really important to get people to make that initial donation/ action, however small to support the charity, and b) to remind people they gave to you when asking for a follow up donation/ action