According to National Safety Council guidelines:

• If you’re over 40, inactive, or have a history of heart trouble, ask your doctor whether it’s safe for you to shovel snow. If it’s not, ask someone else to do the shoveling for you.
• Don’t shovel after eating, drinking caffeinated beverages, or while you’re smoking.
• Stretch and warm up before you shovel.
• Shovel while the snow is fresh and powdery and get started early. Waiting for the snow to accumulate means that you’ll be shoveling heavy loads.
• Don’t pick up a lot at once. Use a small shovel or fill only a quarter or half of a larger one.
• Bend your legs, not your back, while you lift. Let your shoulders, torso, and thighs do the work for you.
• Take frequent breaks.
• Stop before you’re exhausted. 
• If you run out of breath, take a break immediately. If you feel pain in your chest, stop shoveling and call 911.