My left arm is a huge part of my distance throwing. I developed almost a mental step-by-step that I used to run through before it became ingrained in muscle memory:
starting at the reach-back:
1) reach left arm back as far as possible, and right arm back inline with my intended line.
2) focus on rotating hips 180 degrees away from the target when stepping through the x-step with my left leg.
3) swing pivot foot to plant, and focus on swinging hips around, keeping my arms back until they can't stay back anymore.
4) swing left arm down and through HARD.
5) let left shoulder push right arm into the power zone.
6) shift focus to my right arm, pull in tight and focus on the hit and wrist angle, squeezing the ever-loving god out of the disc as it comes through.
#6 is probably the most difficult, because if you do the previous 5 correctly, you're coming around pretty damned fast at that point, and getting the timing right is hard.
The way I throw is probably best described as a 'Swedish Lever' style throw, but it works for me. If I don't focus on my left arm/shoulder when starting to rotate, I end up strong-arming pretty much every time.