A lot of big charities in the UK employee "clipboard-nazis" near busy shopping areas to get people to sign up and make you feel guilty if you don't. These people normally earn 25 GBP per sign-up (which takes on average 10 months to recover iirc). Other charities spam you with mailings.

Now clearly this works: they are raising more money this way, but the side effect is that people like me refuse to give to a charity that hassles me every time I go to the supermarket, and spams me every time I give them money.