What Creats Blow By & How can you Reduce it ???

Hello everybody,

I have a friend who just bought a truck that has heavy blow by in it. He got it from the most famous auction site. When we remove the oil cap there is white smoke puffing out like a choo choo train... and when you remove the oil dipstick you can see white smoke coming out of that too..... the exhaust has some smoke when the truck is cold but that kind of goes away as it warms up. The truck does run excellent but the blow by is there.... What really causes blow by??? and is there any way to stop the blow by in this truck or reduce it???

The truck is:

1999 Ford F250 XL with 126K miles on it PowerStroke Automatic.

Thanks !