Switch to 5 GHz. You'll need a new router and WiFi adapters, but they're available. You won't have quite the range, but even business locations with dozens of 2.4 GHz networks (e.g. Physician office buildings) will have 1-2 5GHz networks total, and most off the shelf consumer laptops don't support it right now so adoption will take a while.
Part of the problem with 2.4 GHz in big buildings is that it passes relatively well through building materials, so that in a large building you get a lot of noise from equipment owned by other residents.
Someone measured the difference in transmissivity for different materials at 2 and 5 GHz (http://www.ko4bb.com/Manuals/05)_GPS_Timing/E10589_Propagati...) - the difference is quite large for brick and cinder block; unfortunately, they didn't measure insulation foil, which I expect would be quite a good RF shield, especially if earthed.
So if everyone switched to 5 GHz, the interference situation might still be better than if everyone was on 2.4 GHz.
Of course, being an early adopter of 5GHz probably helps with the other problem in apartment buildings, the 'prisoners dilemma' type problem where everyone who knows how ups their transmit power because of the noise, making the problem worse for everyone else but temporarily better for them, when if everyone lowered their power, everyone would get better speeds (exactly analogous to how everyone ends up shouting when having a conversation with the person next to them at a party, when if everyone talked quietly instead communication would be more successful).
Not necessary. The 5ghz band has a lot more channels available (21 in the US), and all of those are 20mhz wide with no overlap. You would have to live in an extremely dense apartment building to start to run into interference issues.
5 GHz also effectively has a shorter range than 2.4 GHz, particularly with building materials in the way. That gives more available capacity, and even without that it's likely to be at least another year before 5 GHz becomes standard in consumer laptops, plus most people aren't going to be replacing existing equipment immediately.
If possible increase the transmit output milliwatts, to "shout over" the other signals on the same channel.
Increase the beacon period of your router/AP to the fastest it will go, usually 20 microseconds. I think this is the best solution of anything you could try.
Maybe play around with the DTIM interval but I'm not sure what a good number for that is; too low supposedly drains batteries of wireless devices and too high causes other problems.
Max out RTS/CTS but you may lose overall throughput but that's no loss if it suck already, in for a lamb in for a sheep.
There are other settings too most explained in the help section of the AP/router help menu. They can be obscure but fiddling with them can help your AP signal a lot.
I actually design wireless devices for a living and in my opinion, this is mostly bad advice.
The best bet it to switch to 5GHz, but that isn't practical for use with mobile devices.
Option B is to turn off the higher speed modes like 802.11n. Just try to stick with 11g. If you are still having connection problems, maybe switch down to 11b, but that will be real slow even when it works.
If you use an access point with detatchable antennas (which is actually most of them if you open the cover), then maybe replace the omni-directional antenna with a high-gain directional antenna (also known as a patch antenna). Then you'll need to point the antenna at where you sit.
Another (probably lousy) option to try (but very quick and cheap) is to move off of the commonly used WiFi channels (1, 6, 11). They are commonly used because they don't overlap with each other. So maybe try 4 or 9. The downside is that you will get interference from two commonly used channels, but the overall situation may be better than what you've got now.
I've had great success adjusting the beacon period to 20 milliseconds and using 2.4GHz.
Although I do agree with your advice about channels I often get relatives to do that as they first step in troubleshooting problems with their home wireless network.
People have all those adjustable router options they may as well try something rather than default settings. You don't have to design wireless devices for a living to fiddle with a few settings.
You say "switch to 5GHz, but that isn't practical for use with mobile devices" -- is the reasoning that the 5GHz signal doesn't penetrate the walls enough as you walk around with the device?
That is a factor as others have mentioned, but mostly I meant it is just that many mobile devices don't support 5GHz. And you can't just add a cheap USB dongle. That is starting to change though, so check the specs on your devices.
