Keep your back straight and vertical. Divide large loads into smaller loads. Use a device like a hand truck or luggage carrier. Get plenty of rest. Avoid stressful situations.
Stress goes straight to the weakest link.
In this case, it would be your back. A brief nap at home or work will help. Sometimes, at work, I'll walk out to my truck, put the seat back, and take a 10-15 minute nap. I also take short walks about the company compound - relieves stress from my back. Don't sit for prolonged periods of time - sitting is hard on the back.