That's my cutoff. That pretty much results in "Sorry, that's my seat and I'm not interested in swapping". Once the offender is out of the seat, I also mention I might have considered a swap if he/she hadn't just presumed to take it. Hopefully it's a learning experience for them. I've only had to do this I think twice, luckily, in my years of flying. A couple of others seemed to have honestly misidentified their assigned seats (or are good actors).

Since I switched to being an early boarder, I have had no seat poaching issues or overhead issues, so I will just continue to board early. Recently making 1K again, I will be boarding even earlier. But when I used to board toward the end and someone was in my seat, half the time the person had just made a mistake and moved. A few times I was asked to move to be close to a child or something and offered an equivalent seat, and did so because they asked nicely. But if they did not get up when I told them I wanted to keep that seat, I did not argue about it, I just asked the FA to take care of it, and they did. I do not recall any FA ever telling me to just take another seat, but if I had (assuming it was not a complimentary upgrade), I would have filed a complaint and given the name of the FA. I can be made happy again with lots of miles.

Serious question: for those of you who don't mind swapping like-for-like if asked nicely, but object to someone "poaching" your seat, what is the best way for the person who wants to swap to ask? You want them to wait until you are fully settled in and have unpacked your bag, etc.? And if not, where are they supposed to wait to pop the question just as you arrive at your seat?

Obviously a poacher who has fully settled in, unpacked their bag, used the pillow, etc. is very rude / trying to make it hard for you to say no. But on the flip side letting the original person in the seat fully settle in before asking (e.g. waiting until boarding is complete) seems like a mistake too.

The one time in the past decade I've asked someone to swap (me + partner separated due to irrops), I just "perched" in his seat (bag on lap, didn't take my coat off) and when he arrived I just said "hey would you be willing to switch for another aisle further forward so I can sit next to my partner?" and he said "yeah no problem", took a look at my boarding pass, and then went and sat in my assigned seat. (I did make sure we were offering a seat further forward in the plane since most people prefer that, and obviously I would have moved without fuss if he had declined.)

Was I DYKWIA? What is best practice here?

All interesting. If I'm on a 737 or equivalent., I always try to book 1A or 1F. Little more space, easier to get to the head without bothering your seatmate. Usually say no to requests to swap unless a kid's involved. And call me old-fashioned but, when traveling with spouse or kids, I always try to buy seats side-by-side. If I cannot do that, I would never ask someone to swap seats.

Our practice is for the adjacent partner to ask. So if my husband and I are split, when the 3rd party arrives next to us, we explain that we are split and would they consider swapping with the other. This is of course if we are both already part of partial pairs and trying to get a pair together, as in that situation, either of us could move.

I never poach someone else’s seat. I board, then I approach the person who is sitting in the seat I’d like to sit in, explain the reason, and hope he/she is willing to change.

While infrequent, that's the way I've done it, for the same reasons.

I've also proactively offered my seat when I've seen a couple being separated. Biz travel is different from personal travel.

So a couple thoughts: This is as someone who is virtually always flying solo and strongly prefers a window (I get claustrophobic and anxious in an aisle if the flight is more than a quick up-and-down, even more so if the window dweller pulls the shade). If for some reason I'm stuck in an aisle (95% of the time because of a late IROPS rebooking and that's the only J/F seat available, the last 5% due to a late battlefield upgrade I'll board and take my assigned seat but not get too comfortable. Once the person in the adjacent seat shows up before they step in I'll ask "would you prefer the aisle?" I can think of only one occasion where the answer wasn't "yes, please" -- in those cases, I'll quickly slide over into the window and the newcomer drops themselves in to my set. If the answer is "no" I stand up, let them slide in and reclaim the assigned seat.

If I were trying to seat with my wife (who, lets face it, we see enough of and a few hours apart isn't going to kill us -- actually for our last few TPAC/TATLs we've been in front/behind in J anyway -- I'd first try to figure it out before boarding if the GA was willing to play along (I've people-watched a few "would you mind paging whomever is in xxx?" with about a 75% success rate, at least on the page not sure about the ensuing negotiations) otherwise a tactic I've seen others use is that either one half boards and the other waits or both board, in their assigned seats, and then as soon as either the neighbor shows up but before they take the seat "Hey, my husband/wife/side hustle is in xxx, would you mind swapping so we can sit together?" -- can't say I've seen that one rejected and it also is twice as likely to succeed because if #1 says no, the other person can still ask #2.

Though "perching" may temper things, my general attitude is that if you're in my seat preemptively you've already put me in a bad mood and I am not inclined to reward that presumptuousness. I had a relatively long CR7 flight a few years ago where the person from 1A "assumed" I'd be OK with giving up my 2A for a bulkhead. I wasn't. I needed to get work done and wanted my laptop and other resources under the seat in front of me. "Just take my seat -- it's 1A" the poacher replied when confronted. "Sorry, I need to get to my bag in flight" "So do I", she said. "Will you please move to your assigned seat?" I replied. "But there's no room for my bag in 1A" she retorted "Nor mine" I replied with a dead stare. I was about 2 seconds from hitting the FA call button before the situation resolved itself (maybe I'm even odder than I think I am but I have passed on booking flights where the only seat avail for selection is a bulkhead...and if I know I'm going to be stuck in a bulkhead I plan my boarding much differently)

Applause worthy, kudos!

I've had to do this (rarely) when traveling with family since we tend to book last-minute mileage tickets.

I don't sit down in the seat I want - that's a no go for me. I will stand up at the seat (so as not to block the aisle) and when the person arrives, ask gently if they would be willing to switch to another aisle seat on the plane (it's always an aisle seat since my son stalks the window seats...). That's the other thing, no one really wants to sit next to a five-year-old (or younger), and I've never been turned down...

BUT, I will never sit down in the seat without first asking.

i don’t really agree with this. I basically have only ever asked to switch to sit next to a companion and I always will pick an identical seat - aisle or whatever. If I happen to board first I’ll usually sit in the wrong seat but have my bag in my lap and not settle in while trying to make eye contact. I’ll then quickly say would you mind if I sat here my seat is this one (again it’s generally across the aisle or right in front or back of where I am). This has only happened one or two times but never been a problem. I feel like this is easier than asking someone to switch once they’ve settled in, put things away, etc and less disruptive to the overall boarding.

edited to add: just got to the bottom of the thread and saw the discussion on perching. Count me among the perchers!.

According to multiple people above, the DYKWIA in this case is the old man that seat poached without asking.

I am surprised you put your laptop in an overhead bin. In my 20+ years of traveling, I lost count of how many times I watched another traveler lob their heavy roll-aboard in to an overhead bin and in to a fragile laptop. Sometimes you hear the crack of a screen, sometimes you don't. But either way, the laptop almost always loses this fight, and I would imagine the damage isn't always discovered right away. In fact, Ive watched some flight attendants ask people to hold their laptop bag at their seat (mostly because they have seen this too) to avoid this happening.

And overhead bins are cleaned far less frequently than carpets, if at all. The bins are actually not even part of the cleaning SOP, other than lose items, so I would recommend you reconsidering your logic there.

Best poacher I ever had was when I had an old-UA slanty middle business class seat LAX-LHR. Got to my seat and a woman was patiently standing in the aisle and before I could say anything asked "Would you mind swapping seats so I can sit with my husband? My seat is an aisle." I almost hugged her. I've also arrived at my seat to find someone fully camped out and/or who wants to give me a window or middle and I politely decline with a "No, I'm sorry, but I really prefer the aisle and made sure I could get one when I booked."

The couple times I've wanted to swap, my m.o. is same as MatthewLAX's. Hang out but don't sit in the chair. I never ask for a swap unless I am giving them an equiv or better seat in return, and, frankly, my wife and I survive just fine for a few hours being ten feet apart so we typically don't bother. I have to get up a lot and hate bugging people, so if I am in the window on a long flight I've asked before if the aisle person happens to prefer the window (surprisingly at least 50% were thrilled to grab the window), but I don't presume or care if they decline and I would never ask if they're settled in and comfy already.

This recently happened to me which ended up being the best "seat swap" I have ever had. On a EWR to SFO flight my PPs didn't clear (of course) and I went to take my original seat. Per usual I boarded late to find a women in my seat with my same seat assignment. I walked back to the FA told her what was going. She then seat me out the jetway to the GA. The GA was initially pretty freaked, as was I as I was told the flight was filled, but somehow the GA came up with a F lie flat seat and cleared my PPs. All well that ends well.
I guess this would be another reason as to why to board early (I don't) per other above comments, he who has possession of the seat first may ultimately be the one who keeps the seat. I could have ended up being bumped to the next flight had the GA not found a seat for me.