Everybody has their own method. Karl's is a little different than mine. Find which one works best for you.
Here's how I do it. I don't keep my back straight, I bend forward as far as I possibly can, my front knee is touching my chest. I bring my back foot up until I'm almost sitting on the back of the ski. Then I stretch my arms out as far as they will go. Then since I am left foot forward I have the rope on the right side of my ski so it wants to pull me over to the right so I lean a little to the left.
The reason all that helps me is if I am stretched out as far as I can, all the power of the boat goes to the ski. If I have my back straight or my arms bent, when the boat pulls, it snaps me forward. But if you are stretched out, then you are like a solid object.
When the boat starts to pull, push down on your back foot to get the ski to plow. It's kind of hard on your back and once you learn, you'll do it less and less so you don't wear out your back so fast but it really helps keep you stable when you are learning. After that, like Karl said wait a few seconds and slowly stand up.
Wow that was long winded. I hope if helps. To make you feel better. It took me 13 tries before I figured out what works best for me.