Strategy to improve seats on a flight?

The issue is that in order to give you a specific seat, they need it to be free. If the better seats are already given away, normally the people who have received a seat have this printed on their (e-) tickets. Moving someone away from their seat because you want it is very tricky.

So the best strategy would be to get a better seat in the first place instead of upgrading. Early booking, higher loyalty program rank and online booking are ways to do so. Some loyalty programs let you specify preferred lateral locations, too. Choosing an aisle seat is often better than being wedged into a corner. I prefer a window seat so I can lean against the wall and sleep.

The next step, if the above does not work, would be to check in as early as possible. Often you can pick your seat when checking in online and the sooner you do that, the better it is.

The next level would be to ask at the check-in and then again at the gate to make sure that in case there is a no-show, they would give you that seat. The issue with that is that you will be essentially the last guy getting on the airplane and then also likely have less chances to put your luggage into the overhead locker since it might be full already.

It surely differs by airline, but I would assume that in most cases the person who checks you in is not able to move someone else off their reserved/assigned seat without the approval of a supervisor. So unless you can give a good reason to get for getting an extra legroom seat (like a broken leg, a baby with you etc), it might be difficult to convince someone to change the seating order for you.

If there is no way to reserve your ticket at all until you are at the gate, the goal is to be the fastest. Check in early, stand at the gate. Having kids that can run ahead and grab a seat also might be a good strategy.

Almost always, I find the best approach is "gradual improvement". Almost all the seating changes you need to make can be made most effectively through the airline's own website:

1. Firstly, always check if you can pick your seat as soon as your ticket is booked. Most full-price airlines, typically if you have some frequent-flyer status or a higher-priced ticket, will allow you to select your seat as soon as you book. Use seatguru.com or similar to find the best available seat, and pick it.

2. Some airlines (for example, British Airways) allow you to pay extra, either at this stage, or closer to the travel date (sometimes these paid seats open up 48h or 24h before travel) to book exit rows and other desirable seats. Often the rules for when things open up are a bit opaque, so keep checking, especially as you get closer to the day of travel. I have also used expertflyer.com's "seat alerts" in the past to notify me immediately once seats become available (of course, you have to pay a nominal fee of $1 for the worthwhile ones, but they have come through for me on occasion). Of course, only you can decide if paying for a better seat is worth it.

3. Especially if you haven't been able to pick a seat beforehand, check in as early as possible - even if you can't print the boarding pass right away. Obviously this will allow you to secure the best seat you can at this stage.

4. It may also be worth asking at the airport check-in desk, or at the gate. My experience, though, is this is generally a waste of time unless you get lucky (such as someone else having been upgraded or cancelled their booking) - leaving it to this stage, as some inexperienced flyers do, to pick your seat for the first time is definitely a mistake (I was once flying with 6 such people, one of whom proceeded to have an argument with the check-in lady for 20 minutes as she wanted us all sat together. This is completely the wrong time to resolve this, as most passengers already have an assigned seat).