There's also another way besides the one Hive account@dlwagner posted (not that he's wrong, just another way). You can also use blocktrades.us and then send btc directly to a user on steemit, and they receive it as steem. Once they receive it they can power up.

Basically there are two ways to get STEEM power - by either buying it directly with USD/BTC, or by earning through payout/interest.