I don't see how you can do that without making it centrally controlled and acting like the fed (printing more or less based upon an inflation target of 0%).
My idea (working name RevCoin) is to mine an exchange rate for a currency basket into the blockchain. Miners can use any quote source they want, but relay nodes will only propagate it if they agree the quote is reasonable (preventing miners from gaming it). Heavily smooth the value so that the price can float to let market makers and speculators provide liquidity by trading short-term fluctuations, but over the long term the coins generated per block are adjusted to keep the exchange rate flat. As the number of coins in existence grows, the smoothing-time will also grow to let the price float more; when the BTC market cap reaches the scale of fiat money supplies, it will be essentially free-floating.
The idea is to keep a stable value and prevent speculative "to the moon" fluctuations until it's large enough to stand on its own. If it fails, of course there's no way to deflate to compensate (it will reset the target and recapture control at a lower level), but I think long term success is more likely without the current wild value fluctuations.
See also: EnCoin, GEM.