After waiting several days for the paint to dry (I use PaintStiks, which are supposed to dry in about 24 hours, but this isn't always the case--humidity and the specific color are just two factors that influence drying time, and the Ivory Black I use on the sides takes a while), which I know isn't like waiting months for wet oil paint to dry, I varnish.
I'm still looking for an environmentally-friendly varnish, but until then, I use Kamar Varnish and it protects the painting from bugs, dust, etc. once it's dried, plus it gives the work a nice, almost "wet look" shine.
I varnish a portion of the back, too, where I put my signature, chop imprint, and date. The varnish protects the un-gessoed canvas so that the ink from my pen doesn't bleed through into the artwork on the other side. Theoretically, it shouldn't bleed through without the varnish, but I like to play it safe--it's a quality assurance thing.
The torn and flattened paper bags I use as tarp underneath the painting have been with me for about three years. When I first started, I would tear them up, use them, and then recycle them. But then I realized that they were getting varnished and heck, why not reuse them?
In a few hours, the varnish will have dried enough for me to move the painting to a drying area where I let it "fume" for several weeks. It actually takes that long for it to be completely dry and to stop smelling all funny and stuff.
Okay, I've got to get back to work.