My wife and I currently have the following cards:

-CapitalOne Venture (applied & received 2/11)
-Chase Sapphire (applied & rec 9/25/11)
-US Airways MC (applied & rec 9/30/11)

How does churning work? Do you have to cancel the credit card and wait a certain time before applying for another deal? How do you know how long you have to wait?

Lastly, my wife got a mailing from Southwest saying she was pre-approved for the Chase SW Plus card with 50k bonus miles. How long are we supposed to wait until we can apply for a new Chase card?