Hey RickyZ,
Kudos to you for wanting to help others with your blessing.
I have to admit that I get really annoyed by charities/ministries that actively solicit for donations, but then don't bother to acknowledge or respond to gifts--particularly when I have to be the one to later contact them again for a receipt! You'd think that would be the very least they could do, especially for the purpose of taxes and keeping records.
However, one thing I know God has convicted me of through this are all the times when HE does something for me, and I never acknowledge HIM or tell HIM thank-you--talk about hitting close to home!
I don't know if this will help any, but I've gotten to a point where I mostly give to charities that I have had close affiliations with (such as my old adoption agency), and know from experience that they they're good stewards of what they're given, so it doesn't matter to me if I never receive a thank-you. I also only donate to places that will at least automatically give some sort of receipt, and not make me have to hunt them down for one.
Also... Are you ever able to work with the same representative for your donations? I give some donations by phone/email and have continued to ask for the same person if possible. I know this might not work because volunteers change hands all the time, but I've been fortunate enough to develop relationships/rapport with a few regular staff members, and they've been really great at responding.
May God bless you for having a heart for those in need.