One thing you can do after draining the tank remove the drain valve and poke around in there with a stiff wire coat hanger. After you done that a few times, put the valve back on with a hose attached and turn the water on with the drain valve open. after the water starts to clear turn water off again, remove the valve and repeat, till you do not get any more deposits.
Another way is to call a pro and have them use Un-Lime from AO Smith to delime the tank. This is a costly option some times its cheaper to by a new water heater.