After perusing AA.com I’ve found that you can sometimes purchase upgrades to First Class at a cost of $30/500 miles if available. Of course there’s about a thousand restrictions.
The main thing I’d like to know is if this is at all possible with an internet-booked flight (i.e. with what the airlines call a Deep-Discounted fare class O ticket) because I think that renders it all moot…
My parents just asked when they got on the plane. Be prepared to stay in cattle class, though: on another trip, I was in business class and it was full; there were no upgrades available.
Agents have a lot of power to do what they want. I have been bumped up several times for free with discount tickets. A very strong tip is to wear nice clothes (at least a polo shirt and nice pants). They generally won’t upgrade someone that dresses and smells like a hippie. They like the First-Class cabin to look nice as everyone else piles so they won’t let the riff-raff get an upgrade when they don’t look the part.
I often pack my sombrero and serape to avoid such problems when upgrading myself, that and to avoid being charged for two seats. 
Seriously, if the plane looks like its going to be empty just ask when you get there. I’ve been upgraded to First class after I got on an empty plane because the stewardess either didn’t want to drag themselves all the way to the rear to check up on me or they wanted to go back there and sleep/party themselves.
You might be able to do it, but you will be a the back of the queue. Ahead of you will be frequent fliers and people who bought more expensive tickets. And they will both be ahead of you even if they ask for a upgrade after you do.
Basically, the answer to your specific question is “no, you generally can’t buy upgrades for deep-discounted tickets”, regardless of whether you booked online through a travel site, over the phone via a travel agent, or whatever. The key piece of information is the fare class for your ticket. On American, according to their “buy upgrades” page, you can only purchase upgrades for “Y” (full fare coach) or “B” (discounted unrestricted) fare classes. These will generally be significantly more expensive than the deep-discount classes (“L”, “U”, and “T” on Delta – don’t fly AA often enough to remember their fare ladder). You may, if you believe there’s sufficient unsold inventory in F to make it worthwhile, call the airline and see if you can upgrade your deep-discount ticket to a Y or B fare class (at whatever the current fare difference is), then turn around and try to purchase the upgrade based on your higher fare class. But knowing whether it’s worthwhile is generally going to depend on being able to determine the available inventory in each fare class and current fare for each. There are tools to make that easier; if you’re interested enough I can offer some suggestions.
Most of the answers you’ve received so far are to a different question really, i.e. “How do I cadge free upgrades to first class when I’m booked in coach?” It happens, and gate agents do have a lot of latitude once all the folks who’re “entitled” to upgrades because of their loyalty program status, etc., have been accommodated, or when there’s a need to do an op-up (“operational upgrade” – essentially, when coach is oversold and there are open seats in F), but even then loyalty program members are going to get priority. Go ahead and dress well if you like, but I wouldn’t count on that and a smile to get you upgraded.
My favourite trick is to look as tramp-like as possible but actually buy business class tickets, it’s great fun to irritate the other business class travellers like that, particularly if you’re on holiday, quaffing champagne and generally being loud.
Sadly, most short haul flights now there is no point at all in buying business. My local airport is ryanair only(ish), but for some reason most people aren’t prepared to pay the £2.50 extra and get guaranteed (pretty much) emergency exit / front row seats. Fine by me
Penn and Teller suggest that a good way to get free upgrades is to paste a little red foam-rubber dot over your nose on whatever ID you plan to show when you check in, then in that second between handing the counter agent your ID and them reacting, slip on a clown nose.
I have yet to try this. YMMV.
I fly AA approx 2-3 segments/week.
It doesn’t matter how you booked your tickets, but you are right that there are different classes of fares.
You will be able to tell online whether or not you can upgrade it, and for how much.
I am very skeptical about the assorted schemes posted here, and I’d be surprised if any of them work.
In general, AA cares about two things: How much did you pay for your ticket? and Are you a regular AA flier?
Until you get Platinum status (50 flight segments/year) you are not going to persuade the average Gate Agent with clothing gimmicks and the like. There are a lot of myths out there, and an occasional exception to the rule, but in general AA does not make a habit out of upgrading a cheap ticket to first class without a reason. If they did, everyone would get to first class that way.
FWIW first class is not worth much. Meals are served only on longer hauls. The seats aren’t much better than an exit row, in my opinion. I seldom bother to upgrade, and I have plenty of free upgrades available b/c of my flying status.
I’ve only been upgraded to first class once. I was 10 and flying (unaccompanied minor) to Hawaii to meet his new wife and stepkids. It was on Delta and the stewardesses though I was cute 
. :o
You can get a free upgrade to first class (from any ticket grade) by dying midflight.
That said, the flight attendants never bring you drinks. And if you press the call button, they start screaming 
Si - (thinking of changing his name to Hotblack Desiato)
Careful, you’ll have the Inland Revenue after you.
I know one National airline that sells upgrades on board, they do it through the duty free till - since I wrote the software, I know how it works.
They are not really supposed to upgrade pax flying on deep discounted tickets, but the only check is entering the ticket type 
(well they are supposed to enter the ticket number as well, but nobody is going to cross reference it at head office)
I reckon dressing smartly and being relaxed at check in helps for freebie upgrades and getting better seats - it has certainly worked for me.
I bet Hotblack did not have HMRC and Death Duties to deal with. You do actually pay more tax in the UK when you are dead. One day, they will tax births as well, so they can get you coming and going.
Si
Death duties tax your old b/stard’s estate
Where, when and who do you ask for the upgrade, though?
I generally use the electronic boarding pass machines, where I can just swipe my credit card or enter my ticket number and get my boarding pass printed out without waiting on the long line to interface with a person at the counter.
Is that the only real reason to go stand on that line? To ask for a seat upgrade?
(It’s not to check in luggage as there is usually a separate line available nowadays for luggage check-in versus passenger check-in…)
The last couple times I flew they announced over the loudspeaker that upgrades to first class were available for like $100. First ones to the counter to ask for them and pay got them. This probably isn’t an everyday thing though.
The definitive answer to all questions is, “It depends”. But standing in line at the ticket counter probably isn’t going to buy you much.
In this case, it depends on how the airline operates, whether you’re a member of the airline’s loyalty program, and the circumstances around your particular flight.
If you’re asking for an upgrade just because you’re so well-dressed and personable, the person to ask is the gate agent in the boarding area. They essentially own decisions about seat assignments, etc., within the constraints of the airline’s policies. The folks at the ticket counter can usually help you if you’re just looking for a seat assignment, but the gate agent is really the one who can be your best friend or your worst enemy.
If you’re requesting an complimentary upgrade as a benefit of your status in the airline’s loyalty program, there’ll be specific, published procedures for it. In the case of Delta, the airline I’m most familiar with, Medallion-level SkyMiles members can request upgrades at the time of booking, or any time after booking and before check-in, through Delta’s web site or by calling SkyMiles Member Services. You can also request an upgrade as part of the check-in process at the airport kiosks or at the ticket counter (though either is almost always pointless – if you haven’t requested it before then, you’ve got almost no chance). Note that I say “you can request” an upgrade. Whether and when you get it depend on unsold inventory of F seats, how many F seats the airline’s yield management systems say should be held for last minute purchases, your status in the airline’s loyalty program, and the date/time of your request. Assuming there’s available seats in F, upgrades begin clearing 5 days before the flight for Platinum Medallion members, 3 days out for Golds, and the day before for Silvers. Once those are cleared, what’s left are the seats the airline thinks it still has a chance to sell. If that doesn’t happen, the airline will open those seats for standby upgrades shortly before departure time – priority is determined as above. Delta has LCD or plasma displays in its gate areas at many airports that display real-time information on the number of seats in each cabin, how many have checked in, how many have been sold but are not checked in, and how many are unsold. They also show the standby upgrade list in order of priority. You can generally tell from that information how much chance you have. I’ve actually been 36th on the upgrade list for an SFO-ATL flight that had only 2 possible upgrade seats left when I got to the gate – why they even bother showing that far down the list I don’t know, except perhaps to eliminate the questions about whether you got accidentally dropped from the list.
On those rare occasions when all loyalty program members in the upgrade queue have been accommodated and there are still unsold seats in F within a few hours of departure, the airlines will do what Antinor01 mentions – offer paid upgrades to coach passengers. Generally, however, there are still restrictions on which fare classes are upgradeable. On Delta, the deep-discount L, U, and T fares generally aren’t eligible.
Discount airlines like AirTran that also have some sort of premium cabin (AirTran calls theirs Business Class) will also offer day-of-flight paid upgrades for something between $35 and $100 dollars, if they have excess unsold inventory. In the case of AirTran, you’ll get a prompt during the kiosk check-in process asking whether you’d like to upgrade for $50 or whatever the charge is – if so, you swipe a credit card and get your upgraded boarding pass.
If all the planets align right, and even that doesn’t fill up the F cabin, you might sweet-talk the gate agent into something, if you’re wearing your best clothes and say “Swordfish”.
Most of the cases mentioned earlier of folks getting spontaneous upgrades at the gate or on the plane are operational upgrades, where they have oversold coach and have open seats in F. Gate agents have a lot of latitude here, but are still going to favor loyalty program members when they have time to figure out who’s who. Size of your party may also play a role in who gets these – they’ll want to move the fewest number of parties, so if there’s two available F seats and a couple of parties of two waiting for coach seats, two people traveling together are probably more likely to get bumped up to F than two individuals, all other things being equal.