For AA, you want the Starwood Preferred Guest American Express card. The points can be transferred in blocks of 20,000 with a 5,000 bonus to AA (and a bunch of other airlines to boot), so it's really a 1.25 miles per dollar, which beats the Citi AA card at 1 per and has an annual fee of only $45 (waived the first year).

There's an awesome offer that just expired for 30,000 points with sign-up and no annual fee for the first year, which you should be able to find here. It's technically expired, but as long as the application is still working, you can generally squeeze in anyway (and at least one person has had a CSR at Amex confirm that).

That's the card you should use, but as long as you're going for AA miles, you might as well sign up for one of their cobranded credit card offers too. I think the current one is no annual fee for the first year and 35,000 bonus miles. Look in the "other credit cards" subforum here. You can cancel that one before the annual fee hits.

If you want UA miles, I think you're probably stuck with the UA Visa, which also has a good bonus offer (so no reason not to go after that at least once too, as well as the cards/checking accounts for continental, since those miles will eventually be merged into United), but its fee is more than $50 for the good one (after the first year, if you manage to find an offer with bonus miles and no annual fee).

You should look around this and the credit cards subforums for more details.