It depends on the amount and the context.

I mostly work in millibitcoins and have done since 2011:
    e.g. "34.6 mills".
If an amount is a whole number of bitcoins I'll use this:
    e.g. "4 BTC" or sometimes "4 bitcoins".
For tiny amounts such as fees I prefer microbitcoins and satoshis:
    e.g. "25 mics" or "2500 satoshis".

If the value of a bitcoin rises sufficiently, perhaps to around 10 USD/mBTC, then I'll probably rebase to mics.