Agree with daredevil123. As a volunteer on the ground, i notice that apprehension when soliciting for donations, i think we have to be very mindful of the words you use in order to get a better conversion rate.
After discussing with some of the bros and listening to their feedback during dinner, i've come to a conclusion that it's better to tell would be donors upfront that we would like their help to take their photo for free to send our moral support to the people of Japan. The donation should be the secondary objective and that any amount of donations would do but donations of $10 and above would get them a 4R print. By this time, they should have already warmed up to you after showing them the lovely photo you've taken. It's alright that they do not donate and we can tell them about their photos being uploaded on Facebook and we can get more publicity on the event.
Maybe we can have the other volunteers share what works for them for newbies like me doing this for the first time...