Churning is one of the most wonderful activities known to humanity. In churning, you apply for and get a credit card which are offers bonus miles (say 25K), use the card, get the bonus, reapply, use new card, get another bonus, etc. Then use miles to fly 1st class to Paris and enjoy.
Unfortunately, while churning was pretty easy until eighteen months or so ago, the credit collapse has made it pretty tough, and a lot of FTers have stopped for a bit--others fight on!