I keep a folder in my filing cabinet named (Quilting Donations) in that folder I drop receipts that are for materials I use for donated quilt items. I also write up a regular 'bill of sale' style invoice documenting the quilt- size, fabrics used, theme, charity donated to, date made, date donated, and a picture of it. I keep a small journal in my purse- when I spend money I know will be for a specific donation project I write down the project and keep a 'running tally' of the expenses. I save all of my fabric, tools, patterns, materials receipts regardless of what they are for, so, if I use fabric from my stash for a donation quilt I can easily look up how much it cost, how much of it I used for the donation quilt. so, when I start a new one I can easily figure out the cost involved. I do have an accountant do my taxes every year- he is always very happy with my 'organization'
I generally donate between $600 & $700 in quilting every year- and depending on the place I donate to sometimes I get a tax receipt from them for the quilt- but if not I have a good record of it on my own.