If you have separate email addresses (and who doesn't or can't), the Netflix idea still works for a second account. I have separate accounts with AA and UA and have earned miles from Netflix in both accounts. Yes, I'm making double payments for the same service, but I'll cancel the UA one at the end of the month (I just got it in late April) and keep my AA. You could conceivably even do this with another email address and AA account number (perhaps for your mother) to get another bonus with AA, but I do not know the hassles of combining miles from separate AA accounts so that might not be worth the effort.

Earning for cell phone contracts through most of the shopping malls generally requires new contracts so I'm not entirely sure you can seamlessly move from your current AT&T contract to a new contract so as to earn AA (or UA) miles. You do have to buy a new phone and follow these T&Cs (from the AAdvantage shopping mall):
Don't know what those "other restrictions" are, but you'll want to be clear on those before spending your money. Perhaps you could let your current contract expire for a week and then take a new phone and plan, just to be sure.

As JPG3392 notes, the restaurants in the airline dining programs are almost exactly the same across all programs. If you register a card with a program that was previously in another program, the card is automatically unregistered from the first program--in fact, you'll get two emails, one from the program you registered with confirming the registration and one from the other program confirming the removal of the card. Keep in mind that the programs require that you maintain one credit card registered with the program at all times. There's no limit on registering and unregistering, however.

In addition to all the earlier suggestions, you should also check hotel points programs. Two reasons. First, many hotel programs allow transfer of points earned at the hotels to airline programs although, with one exception, not at a 1:1 ratio. The ratio is generally more like 5:1. The only exception is SPG which generally does 1:1 (although, alas, not with UA where it's 2:1) but will do 1:1.25 if you transfer in blocks of 20K Starpoints. Unfortunately, regular earnings rates with SPG can be a bit low unless you earn status and carry an Amex SPG card. I'm going to assume that you don't do more than 10 nights per year in a hotel--the level at which some programs grant you first-level status and an increase in your points-earning rate. If you're paying for more than 10 hotel nights (or could and be reimbursed), getting status might help boost your earnings rate. For more on status (and the important distinction between "nights" and "stays" for this and other purposes), check the hotel program's website or the specific hotel forum here at FT.

Second, many hotel points programs also partner with airlines to offer miles earning opportunities just for staying at the hotel. Right now, Hilton, which probably implements this feature the best of any program, has a double points/double miles promotion running through the end of June. You have to choose double points or double miles for the duration of the promo, but you can switch the airlines with which you'll earn the points throughout the promo. UA and AA both participate in the HHonors program and each normally earn 500 miles per stay (1000 per stay with the current promo). There's a lot of esoterica with these kinds of promos--registration is generally necessary, points are generally earned per "stay" and not per "night" (which means you may need to learn the value of "hotel hopping" and "mattress runs"), and you'll generally need to book through the hotel chain's website and not a third-party website, such as Expedia or Travelocity--but if you're doing hotel stays anyway, this is an opportunity that you should not miss.

Sorry for the second long message, but I figure this stuff is a whole lot less complex than the medical texts you read!