I keep a spreadsheet online, with the number of the quilt, name, quilted or top, where sold (or present), date sold or given. I started out taking film pictures, then there was a large gap where I didn't take any, the last few years I have pics of every one with the digital camera.
I give a number to every quilt or top I make, ones I quilt for someone else, and ones I repair for someone else. My antique quilts get a separate spreadsheet.