Instead of heaving the dirt over your shoulder or behind your body, why not take a less straining approach? It will save you energy and help you to avoid muscle strain if you move your whole body to where you want the dirt to go, then just turn the shovel handle to let it fall there. As usual, any bending should be at the hips and knees to help you avoid using the vulnerable areas of your back to do the heavy work.
Sources:
Hage, M., Rehabilitation Institute of Chicago. The Back Pain Book: A Self-Help Guide for Daily Relief of Neck and Low Back Pain. 2nd Edition. Peachtree Publishers. 1992, 2005.
Cathy Butler. Personal Interview and Course Notes: Effortless Gardening, as developed by Miriam Levenson, Feldenkrais Practitioner.