Hi Zac,
I currently work for Salem Township Library as a cataloging specialist. We print 1" demco spine labels (from our regular printer) and attach these. We do use clear labels over them, and depending on the width of the book, we try to press these carefully into the edges around the spine on thin books.
I previously worked for Kent District Library doing the same thing, and they use various widths of Scotch book tape. Again, the key was pressing the tape carefully into the grooves along the sides of the thinner titles, and using enough length of tape to make sure it stayed in place.
Hope this helps,