Thoughtfully not doing what they were told saved some lives at WTC. A lot of survivors of the 1st incident vowed, "Never again!" and beat feet the moment they realized something was wrong on the 2nd go around. By promptly and hastily evacuating, they guaranteed their survival.
The Soviets were very keen on PALs because Party, Army and KGB had zero trust for each other and this allowed nuclear control without trust. The Perimeter / Dead Hand system mentioned above for strategic nukes is because there would be no time to distribute codes through ordinary communications links. Remember that the Soviet Union felt that any large war would immediately be nuclear, certainly any that they chose to start; conventional war took up only a small part of their overall doctrine.
Active military units are in a position to track and perform maintenance (if only by receiving new ones from depot and returning old for servicing). Nukes left on the shelf expire for a variety of publicly known technical reasons, some mentioned in comments here.
@Jan. On the money.
@spacemanspiff. The flash can blind you, and yes there is an exposure accompanying the detonation, but soldiers are trained to immediately duck for cover at the first sign of a flash as the neutrons are just that fraction slower. The old 'Duck and Cover' videos show this quite graphically in a variety of situations. If you are too close of course, you do an excellent imitation of a cigarette that is "soggy and hard to light." This is why ducking for cover is so important; people survived in nearby slit trenches at Hiroshima while people in the open much further away were killed. Even newspaper between you and the flash means that the paper is on fire, not you.
You don't need to seal the plastic overmuch. Duct tape would be enough. If it will keep dust and driven rain out, it will keep out fallout. Ideally one would overpressurize to prevent small leaks using a filtration system, but this is not a nicety most people will have time or equipment for. The protection from all this garbage (BNICE, CBRNE, etc.) is Time, Distance and Shielding. What the plastic is doing is creating some artificial distance which helps with certain particles. You will also want to live in your basement, a hole dug under your foundation, or a crawl space protected by as much shielding as you can quickly assemble (i.e. within 1 hour). Think 'playing fort' with furniture, dirt filled bags, and anything else solid. A good book for this is "Nuclear War Survival Skills" http://www.oism.org/nwss/
@Richard and @Doug
Close only counts in nuclear weapons, horseshoes and hand grenades. It's useful to know how to shelter in place if one is unharmed but trapped in a fallout zone. The bigger variable is the immense panic from folks like spacemanspiff who alternate between thinking they're dead already and thinking it's "The End Of The World As We Know It" (TETOWAKI) when it's really a local problem. I'm less scared of the nuke and its effects than I am of the panic.
A good working knowledge of how to take cover can help a lot with the aforementioned explosives as well as the safely hypothetical terrorist nuke. This would be actually useful knowledge to widely disseminate. Look away from the flash, get down and away from windows, move to where you have something as solid as possible between you and the blast (even the lip of a curb!), and protect your head with your arms.
Solidly built furniture can help create void spaces to survive in if a building collapses over you. Of course, it is helpful if one has a whistle to blow so that people know you're trapped. More of an earthquake thing.
If you've read this far, make sure you have a disposable filter mask, a pair of heavy gloves and a whistle around your desk, and sensible shoes if you don't wear them all the time. These inexpensive items are worth their weight in gold for any disaster event.