Watch the Best Straps and Guitars online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
The type of strap you use is also very important. A wide strap should be used to distribute weight evenly. In general even with a wide strap you should try to take a break every 20 to 25 minutes. A guitar that is not too heavy can really help your ability to play longer. Look for guitars the have a weight between 6 and 8 pounds. This will allow you to avoid any problems associated with blood circulation cutoff from a too heavy guitar.