You know how it goes when you transport paintings to an exhibition: frames get dents, glass gets scratches, wires poke holes through canvas -- no matter how carefully you try to avoid it. And when a collector buys a painting straight from the walls of your show, how are they going to get the painting home with no accidental damage to it?
Here is my solution to the dilemma:
I decided to create envelopes for each painting.

My first thought was to use bubble wrap, but skipped that idea as I realized it would be difficult to apply tape to the bubbly side. So what could I use instead? It hit me that I could go to the hardware store and get some rolls of underlay for floating flooring. This underlay consists of thin foamy plastic that is smooth on both surfaces, and takes well to taping.

I tested the idea by cutting a wide enough bit of it, and fashioned an envelope. First I folded it so that it would create a flap that could be tucked into the pouch, and then I taped the sides together to create the pocket:

I used clear packing taped for the taping. First I applied half of it, lengthwise, to the top part, and then I folded the piece of tape and taped it to the part under it (image above). The process was repeated for the other edge.
For the sake of stability, I also taped across the top of the pocket:

Then I cut the corners of the part that is to be folded in:

The design worked, so I was happy. As many of the smaller paintings were of three sizes, I decided to make a template for each size, and decided on cutting the underlay with a knife:


The templates also allow for cutting through several layers of underlay in one stroke.
Here's a neat stack of finished envelopes:

All ready-made, awaiting the inserting of paintings that will be protected from all sides. Putting the paintings inside the envelopes is very easy and a quick procedure. The envelopes can be reused for paintings of similar sizes, of course, and in order to make that easier, I marked each envelope with the size that would fit into it. The envelopes protect paintings of both small and large sizes, as well as both pastels behind glass, and oils without frames. I recommend a larger/longer tuck-in-flap for paintings that are large.
These envelopes will also allow customers to transport paintings safely, too.
Here you can see the securely protected paintings sitting in boxes, waiting to be moved to the gallery:

So, this was my solution. What is your favorite way of protecting paintings?