Standing up in the saddle helps a good deal with your back not in the same position the entire time during a ride. It allows me to stretch out my back muscles. Even standing up for 60 seconds is going to help.
As others have mentioned the wider tires help a great deal as the tire pressure is much less. A 32mm tire at 80 PSI provides as much load support as a 25mm tire at 100 PSI and it is simply that the 32mm tire hold a greater volume of air.