Since the value of Bitcoins most likely will raise drastically I'd recommend using the smallest unit in the currency as basis.
Like for example the Japanese Yen does.
Price Numbers seem to be rather high compared to other currencies, but as seen by example it works.
so 1E-8 BTC = 1 Bitcoin
The only problem would be to bridge the gap aslong as BitCoins aren't as valuable.
There you can throw in filler units:
Like 1 BitCoinCluster for the current 1 BTC setup instead of 10 million BitCoins
and another unit for the transition between the two:
demi- or di- (Bit)Coin - ( demi = half ) which deals with 1E-4 or 0.0001
of the BitCoinClusters or 1000 BitCoins
160 di(Bit)Coins would approximate to about 10 cents/0.10$ at the moment.1 BitCoinCluster = 10000 diCoins/demiBitCoins
1 diCoin = 1000 BitCoins
1 BitCoin = 0.0000001 BitCoinCluster
10.000.000 BitCoins = 1 BitCoinCluster
500.000.000 BitCoins = 50 BitCoinClusters ( upon each successful solve )
The prefixes could be:
BitCoinCluster -> either remain 'BTC' ( so the market dimension doesn't necesserily need to change ( like when 1 BTC suddenly is only 1E-8 of the previous BTC )) or 'BCC' for better fit
BitCoins -> depending on the Clusters either 'BTC' or just 'BC'
'BC' would be a gimmick since it is usually used as "Before Christ" and BC(BitCoins) are before
BTC(BitCoinClusters) so it would make 'BTC' the initial coin into "Christ" and rendering the creator of these namely "Satoshi Nakamoto
" into "our Savior
also instead of solid prefixes something like:C-Coin(Cluster-Coin)
could be used.