That all important thank you letter - as stated earlier, why not ask a beneficiary to write about the impact the charity has had on their life and include this in the thank you letter.  You could even go one step further and ask that individual to personalise the letter directly to the supporter. People don't just want to hear about ''how'' you do your work, or the equipment brought, or the hours spent counselling over the phone, while this is all important - people want to hear people stories.  Stories that tell them the difference their very support has had on someone's life or the animals that were able to find a home etc. 

Some months have gone past (hopefully you will have sustained communication with this individual, they will be receiving the charities newsletter etc)  why not again ask that beneficiary, who appeared in the original thank you letter to write an update on how they are getting on having been supported by the charity, funded by the donors donation and how far they have come.  This shows the donor that their donation is making a huge,lasting difference.  In such a busy, hectic world this will bring your cause to the front of that individuals mind and could well prompt further support.

Going digital - one idea I will be trying soon and I would love to hear from anyone who has already implemented this. Why not ask your beneficiaries/ ambassadors to record themselves (you would need to provide some key messages for them) saying how grateful they are for the donors support and then email it to them (perhaps there's a milestone that's relevant to them e.g. they have been supporting you for a year) - I know I would love to receive that kind of communication.