1. Start slow to warm up: this is exercise!!
Both cardio and strength and that means you need to take your time to slowly bring your heart rate up and give your muscles a chance to get warm.
2. Get a shovel that is the right length for you.
A slight forward lean is ok but you shouldn't feel like you are stooped. Take small scoops if the snow is heavy or push the snow into smaller piles.
3. Take breaks and change sides.
10 scoops left, 10 scoops right, keep things even to keep your back happy. Too many twists in the same direction is what will often lead to a low back strain when you are shovelling. Use the 10 right, 10 left rule to avoid getting to that stage.
4. Ask for help and break the big tasks into smaller ones:
When your muscles start to get tired, this is when injuries are more likely to happen. So stop, rest, stretch, walk around, or go inside for a while and tackle the rest of the job when you feel refreshed.