Here's another medicine ball exercise we've adopted for our Team NordicSkiRacer Thursday night strength sessions that simulates doublepoling. Although the pictures and video show it being done with two people, you can just as easily do it by yourself. Having two people really gets you motivated because you get competitive. Who's going to tire out first? (Usually it's me...)
We're still experimenting, but the two key things focus on are keeping the transverse abdominus tight (go "Hssssssss" to feel those muscles) and to start the action with a sharp crunch of the abdomen. Don't bend at the waist like we did doublepoling in the old days. Instead, use the powerful abs to drive the ball (and your poles) down..
The video - 534KB, Windows Media Video (WMV) format - shows the power that's generated much better than the video stills below. Both Mike Heidinger and Dan Motowski are really wailing on the ball. I think if they stood closer together, they'd actually get a slightly better workout because they'd have to catch a fast moving ball and use their muscles to decelerate it.
We used an eight pound hard medicine ball. See Seated Medicine Ball Toss for Two for another medicine ball exercise.