I teach my sons that if you get arm pump, your gripping the bars with a, "death grip",
Ride relaxed is the first thing! Knees slightly bent, toes a little in, (this helps you grip the tank), knees holding onto the tank, elbow bent and out, head up over the bars, looking down the trail.
When you feel the, " pump", wiggle your fingers one at a time, (when you get a chance)while still holding onto the bars, (like a panio player playing the panio).
I did a hare scramble Sunday, (Pine Grove) and it was a tight track, (not really a woods hare scramble, more like a long MX track with out the jumps, lots of switch backs short sections), any bananna, it was a sit down almost all the time track, I had a hard time getting into a groove, I'm a stand up rider. In my case I gotta practise sitting down and going fast!
Joe :)