I have used cardboard, packing tape and shelf paper. I have even used the rubber glue on a piece of plexi-glass method. They all work for me it just depends on the project, time and process. I mentioned that I use sticky shelf paper (found at wal mart, target, home depot, Wal-greens stores), I have a couple of rolls on hand. It cost about $2 for a roll of 24 feet by 18 inches. It works well for general use. The packing tape, I use for belts because I can use one continuous strip and not pieces.
For me, if I use packing tape, I use name brand tape that is nice and sticky. I will apply a strip to the back side of the belt from one end to another. After I complete this, I will take my glass or wooden burnisher and run it across the top of the tape to make sure that it sticks well to the flesh side of the leather. I trim or fold any excess tape.
The whole purpose for using what has been mentioned or named on your leather pieces is to prevent the leather from stretching out of shape and possibly losing the tooling you applied. One word of caution, remove the tape or other sticky instrument off the leather NOT the leather off the sticky instrument! If you do the latter, it may create wrinkles in your leather from removing it.
Good luck!