I'm sure the actual plumbers will be in here, but for the love of god people, maintain your water heaters.
the easiest and simplest thing you can do is, once a year shut off the water coming in to the tank, shut off the gas or electric, hook a garden hose to the drain, run it outside, and open the drain. let the water come out until all the nasty shit has left. you can literally double or triple the life of your heater that way. The only maintenance I ever did to my old tank was this, and it lasted over 15 of the years I lived here, and the tank wasn't new when we moved in. I do mine when the "spring" change over happens, and I put in a new HVAC filter.
add on to that, if you can, replace the sacrificial anode every 3-5 years. it's usually a separate screwed in rod, but sometimes it's the cold water drip tube. as the name implies, it sacrifices itself to protect your tank's insides. I get all my big parts from http://www.supplyhouse.com and my tank's anode is $22.40, and it's a 60 gallon Bradford White that I paid too much for.