Hydrogen peroxide and a little, plastic scrubber works.

White vinegar will remove this grit. Just pour some in and let it sit until the grit is gone, probably no more than half an hour.

I find that citric acid removes the deposits with very little scrubbing. I bought a big bag of it on the net.

http://www.wholespice.com/display.asp?id=1555

I have tried all the methods, white vinegar, lemon juice, traditional coffee pot cleaners. Like Johnny Apple below posted, by far, the best and easiest is citric acid.
Same stuff that pool places will sell to accompany descaler chemicals.
Ironically, it's the same ingredient (ascorbic acid) sold as vitamin C.
I can find links for some products if you wish. Most forms don't state in their advertising that it is "citric acid", but when I got a decent size bag, that is what it says right on the label. This bag I got was around 12 bucks, I think.
All you do is bring the water in the container to boiling, without the top on with just a couple of tablespoons and turn the power off, and let it sit for around 20 minutes.
You'll see the difference immediately.

Jeff

I use one of those green scrubbers. Go through about one a month. I haven't done the vinegar or citric acid cleaning. As long as the heating element stays clear you're safe.

The minerals get on the sides of the gallon jug and I have no idea how to clean that off. I just get it as clean as I can with the green scrubber. The citric acid sounds like the best bet but I've never done it.

Every now and then I'll take one of those plastic razor blade looking things to it but I really don't worry about it. Just scrub it out with the green thing after every use. The heating element has some minerals on it but it still works very well.

I've had this one for a good five years now and it still works great.

You won't get any of that into the distilled water. Check the top of the jug that screws on. If that has minerals on it it's not working right and those minerals are getting into the distilled water.

There's always a little water left after every use and it still amazes me that people can drink that stuff. Then they complain about arthritis and all that. But everytime I clean it I think about those minerals NOT getting into my joints and arteries.

Doc