You don't always need to play volleyball in order to improve. Another way to avoid burn out is to cross-train. In the off-season or in between leagues, get out of the gym or off the sand or grass court and do something different.
Cross-training not only helps you to keep things fresh, but training different muscles by playing different sports makes you stronger and more fit. Doing different work outs will also help you to avoid injury. Most importantly it will keep you from getting bored and ultimately burning out.
Go for a run, play some soccer or ride your bike. When you decide to return to the court you'll do so with a renewed spirit.
