Hang from a chinup bar.
Lie backwards over an exercise ball or, if you don't have one, a bench.
Practice spine stretching, including t-spine rotations.
Video yourself while shoveling snow and see if, in fact, you are moving the way you think you should be and, if not, fix it. There is proper technique to shoveling - hinge from your hips, "use your legs," as they say, and try not to repeated bend your spine.
(See http://grammarist.com/usage/shovel if you're interested in whether or not shoveling should be spelled with a single "l" or two.)
A shovel with a bent handle that requires you not to bend down as much can help. Here's but one example - you can Google "snow shovel with bent handle" for more:
http://c.shld.net/rpx/i/s/i/spin/image/spin_prod_929934412
Read up on back health. Try Robin McKenzie's, "Treat Your Own Back" as an excellent starting point - it's a short, inexpensive book that forms the basis of much physical therapy for back problems these days.
-S-
