The hardest part is keeping the edge of the mirror from showing up in the picture, since it's tricky to shake the mirror while looking through the viewfinder. This technique can be quite effective when photographing leaves, flowers, or mushrooms. Many fungi, I have found have marvelous translucent quality when lit from behind. A long lens hood averts this problem. In a pinch, a sheet of paper rolled into a tube and slipped over your lens will suffice.
Correct exposure is admittedly, a little trickier with backlit subjects.
With backlighting , many camera meters will be fooled by the intensity of the light, tending to underexpose the entire image. Even with today's sophisticated reflected-light meters, bracketing is still the best bet. There was a day when I toted bellows, flash brackets, and other heavy photographic esoterica on every outing. And from time to time, I still use that stuff. But seeing what a few pieces of cardboard and some aluminum foil can do has taught me that there's still a place for low tech.