When paddling through large river wave, you may notice that the wave as such is pretty easy, but standing through the wave train after that is tricky. I think it's most important to keep the board pointing downstream, and for that, it's crucial to enter with speed and keep that speed. But if the wave train is powerful, this may be pretty difficult.

I also think it's important to keep your paddle in the water as much as possible, i.e. to stick it into each wave peak and pull.

Any better ideas on how to keep standing in powerful wave trains?

You don;t want to be planted on the board. its important to have a loose stand - going toe-heel on your feet, especially on the "lifting" foot. "Dancing" on the board is critical when going through "mixed up" water.

Corran