Just peg it to a major currency. I don't know who could do this though.
I was only proposing moderating the fluctuations, but yes, finding a way to fix the exchange rate would do that.
But that's really just sort of a way of restating the problem, isn't it?
The question is - HOW to do it, and who has the power to do it with Bitcoin?
Write a code that pegs it to USD, then have everyone hard fork.
If you want to know why major currencies are stable compared to minor ones. It's because the corresponding economies/ govts are stable.