I remember hearing a saying "fast arms - fast legs" and while thats true I tend to agree with mkroyer. It's usually best left to the end of a run when you're legs need a little helping hand.
Having said that I find using my arms when mountain running to be extremely beneficial. They help to "pull" my legs up as well lol. However the action is low down beside the body rather than up high or out to the side. Short sharp pumping motion.
For increasing speed I agree that interval training is a great way to improve. I usually pick a landmark and sprint from it to another in the distance - if I am not absolutely dead (not seeing those black spots lol) I will push on further until I do then drop back and recover. These sessions are generally short but they've improved my speed quickly.
If you have hills near you run up those as well as someone further up mentioned they're great for your cardiovascular endurance. Nothing opens up the lungs like running up a mountain!
Good luck
