What is seizing?
Just about the worst thing that can happen to any kind of piston engine. Seizing is what happens when, due to excessive heat, insufficient lubrication, or just being an H2, one or all of the pistons gets stuck in the cylinder in which it's supposed to run.
Obviously, this is bad. It happens mostly on two-stroke engines, because they are not positively lubricated.
In an Otto cycle engine, the space below the piston is not used, so it can be filled with an oil spray to keep the bore lubricated. Two-stroke engines use the crankcase as a holding chamber for the fuel mixture, so they can't do this. They have to mix their lubricant with the fuel. Obviously this means that two-stroke engines aren't as well lubricated as four-stroke ones.
Engines can seize in two ways: while they're running, and while they're standing for long periods. Either way is bad. While they're standing is less bad, because it's not dangerous.
How can I stop my engine seizing?
The two forms of seizure obviously have different causes. To avoid a heat-related seize, keep your engine cool. Run it on good quality oil, preferably a synthetic one, if you can. Keep its coolant topped up, if it is liquid cooled. Make sure the fan works, if there is one. Generally make sure the cooling system is happy; if it's a VW, don't fit silly aftermarket rod covers. If it's an MZ or BMW, clean all the roadkill out of the cooling fins, you disgusting biker scumbag.
The importance of warming up your engine properly before using it cannot be overstressed. Let it warm thoroughly, so that all the clearances can stabilize nicely. Thermal differentials are *not* your friend when you have a piston moving up and down at 5000 rpm, less than a hundredth of a millimetre from the walls of the cylinder; let it all warm up. It'll last longer that way, too.
To avoid a standing-for-long-periods seize, turn the engine over regularly. Let it all run for a little while if you can, even if you're not going anywhere. This class of seizure is usually caused by moisture and stuff entering the engine through the exhaust, so run the engine, let it warm up nicely, then shut it down. Never, ever leave an engine with a spark plug missing; even indoors the bore and piston will have rusted into one lump in a few days.
My engine's seized! What should I do?
If you are riding or driving at the time: Disengage the clutch. Quickly. This will give you back control of the vehicle and maybe reduce the damage to the engine. Coast to the side of the road and try to remember how to breathe.
Once the engine has cooled down, the method for de-seizing is the same regardless of the cause of the problem. The first thing to try is rocking the engine gently. Put the bike or car in first gear and let the clutch engage fully. Rock it backwards and forwards; if you're trying to de-seize a car, you'll need a team of volunteers to do this. Just do it very gently. Sometimes, with mild seizes, this will be enough. Usually not, and it won't work on automatic transmissions either.
If that doesn't work, the next thing to try is the Coke trick. For this you need some cola-type beverage and a spark plug wrench. Simply remove the spark plug(s) and pour Coke into the hole. Wait about twenty minutes, then, with the plugs still removed, try to turn the engine over. You can use the electric starter for this, if there is one. It should turn over very fast and easily, spraying Coke everywhere. Then replace the spark plugs and reattach their caps. You should eventually be able to get the engine to restart. (This method even works with automatic transmissions.)
The reason this works is that Coke contains phosphoric acid, you know, the same stuff that dissolves your tooth enamel. It's enough that it can slightly dissolve and penetrate the seized area where the friction has welded the piston rings to the bore. It also dissolves rust, and attacks oxidation- it's excellent for cleaning alloy wheels and for scouring rusty gas tanks.
If that doesn't work, then you need PlusGas. PlusGas is like WD40 on steroids. Spray an enormous amount of it into the spark plug holes. Leave it there a long time. Really, a long time. Two or three days ought to do it. Then remove the exhaust (because you don't want it full of PlusGas) and rock the vehicle to free the pistons. (If your car is automatic, you can try the starter in short pulses.)
If that doesn't free it, it's time to take the head off and start pounding the stuck piston with a piece of wood.
Obviously, this is not recommended for new vehicles; if you have roadside assistance available it's probably a better idea to call them, as you could well cause more damage doing this. You never know though, it could save your life.