I use oven cleaner, sponge w/ scrubber top, and a hose. I know people use rubbing compound to get the enamel nice and shiney. My kettle is relatively new, so I havent felt the need to do the enamel up because it still has its shine.
For the grates I just do a high temperature burn to carbonize everything and take the grill brush to it. Then wipe it with wet cloth and let it dry. Then apply oil to it.
The inside enamel I used oven cleaner to get all the scaling off so it was like new. The bad thing with this is you lose your seasoning. The good news is, the kettle looks almost brand new.