Caused by humidity in the air, a combustion process byproduct, and condensation on the interior exhaust system walls when the air cools down and the air can no longer suspend moisture within the air mass.
Reheating puts the water back into the air. When the heated air cools down again, visible water vapor occurs.
You can fix it by not allowing the air to have humidity, not allowing the temperature to change, or never stopping the engine which allows the exhaust to cool down. Or, you don't run the engine to heat the moisture in the exhaust.
It is rather difficult to fix something that is not actually broken.
Each exhaust has a water drain at it's low point. Make sure that drain hole is clear. Don't put water into the pipes when washing the bike. And, depending on the outside temperature, run the bike long enough to achieve full operating temperature and run it long enough and fast enough to thoroughly heat the entire exhaust system. This will minimize the water vapor exiting upon next start up.
Cheers,
An aside:
I learned about this in high school physical science class. Is this stuff no longer taught anymore? Just wondering, because this topic comes up frequently in this forum.
