You will always get a build up eventually, how fast depends on the hardness of your water. I fill/soak mine in straight white vinegar, which I reuse, every so often but I don't worry about build up too often. My water isn't very hard. Too much build up will prevent it from heating properly and it won't work.
If you are looking for some way to totally prevent any build up, I don't think there is an answer. I'd be glad to hear differently, tho!