I (and my wife) have been churning both AA and UA. Each have 20K miles/no fee offers right now.

AA I cancel as soon as miles post, wait 60 days, then re-apply.

UA I cancel after 6 months, I got 2 open at a time

I also opened DL, CO, and US this year but I'm not churning those, yet. (I heard DL is not possible though)

I also got 40K UA miles for opening a Wachovia home equity line of credit, which is extremely useful.

I don't care about the effect on my credit score since I'm all set mortage or loan wise. It could have an effect on my insurance rates though.

Every time I 'cancel' I just call the number at the back of another Citi/Chase card and ask them to combine the AA/UA into it. It makes it painless. Side benefit is ever increasing lines of credit.

Citi offered 0% for 12 months, they offered my wife 2% for life of balance. They just sent us checks, which we put in a CD/on mortage principal. The 2 caveats: 1) you can never be late on a payment, otherwise interest jumps to 32%, 2) you cannot use the card for new purchase (which are at 17.5%)

For actual spending I use my Amex SPG (or the Citi/Chase that give 5% for gas/groceries/pharmacy)