A couple packs of B.B.'s in the tank, with a bit of gas or something less volatile. Slosh around. 1/4" nuts or something a little smaller works well too. The sharp edges on the nuts knock loose crusty stuff. Use a magnet to make sure you pull all of them out. I've made a sling set up from ratchet straps to suspend the tank. Gives lots of mobility for rocking it around. It beats having to get helper to assist.