I have had the same problem, gave to Crisis at Christmas then last week they were on the phone asking me give by direct debit. I declined because I thought it was cheeky and I don't give details on the phone, two days later I got a letter from Crisis asking me to reconsider. In November I gave to three charities instead of a birthday gift for a 90 year old, on the same day last week I had letters from the three charities suggesting I donate very large sums of money. I think once they establish that you are prepared to donate they then set about raising the bar higher each time.
The man phoning from Crisis told me they were trying to raise an extra £600,000 and the fundraiser would get £115,000 of that money, I rest my case.