The propane company I use I hold a great loyalty toward. Two days before Thanksgiving several years ago, when I was remodeling my kitchen, the propane company owner, a friend of mine, and I put the finishing touches on my kitchen, which included putting in and hooking up my propane stove top. The propane company owner changed out the gas heads on the stove and made sure it was working properly. Great customer service, at no extra charge, for someone whose main job is to deliver the propane. He's got my business!
Yesterday we had a propane delivery. Before the owner left he took a look at the range top and the water heater to make sure that they had good flames. He also asked when the last time I flushed the hot water heater was. I actually do this a couple times a year and figured then was as good as any. Some people make this really complicated but it's not.
You hook a hose to the hosebib at the bottom of the tank. You open the hosebib up and let water come out. You don't have to completely empty out the tank. You also don't have to close the valve at the top. You just let water run for a short time until it runs clear. In my case, the first cup of water looked nice and rusty. After that it was clear. I probably only ran two or three gallons of water out but that was all that was needed. The tank won't need flushing for six more months or so.
It's important to do this for a couple of reasons. First is if you keep your tank flushed then it will last longer because this buildup will rot out the inside of your tank. Second, and most important for preppers and survivalists is this is a lot of good clean water that you have on hand without even trying.