Neck soreness

[JLMitchell]

I've been riding seriously for about three weeks now. I can do twenty-mile rides, and I'm fairly certain that I'll be doing thirty soon (read: this weekend). I've been riding a mountain bike so far, and I've found that my neck and my trapezius muscles are extra sore, and that I've been popping my neck more often than usual. I think I rode on two rides with a backpack, which gave me pinprick pains toward the end, and they even went down as far as my shoulder blades. I lost the backpack and it's been a whole lot better, but I still have the neck popping problem. It sounds like stretching is in order, but is there a common noob mistake in technique that I should pay attention to?

Edit: I'm going on my first ride with a road bike tonight, I dunno if that helps.