When I started my run this evening, I didn't think about my arm-movement. After about 1/3 mile I remembered I was supposed to be moving my arms. I checked to see what my arms were doing, and guess what? They were right in sync with my legs. So, the habit has been formed.
I've been having problems keeping my arm movement in sync with my legs during walking breaks. While running, I keep my arms bent at approximately 90 degrees. During my walking breaks, I've been letting my arms hang down to give them a rest. I realized that with my arms hanging down, they were like a longer pendulum and had a slower movement. If I keep my arms bent at the 90 degree angle, I have no problems keeping them in sync with my legs while walking.
Another thing that helps keep my arms in sync is to pump them with a slight circular rotation rather than straight out and in. I imagine that I'm grabbing the path and pulling myself faster on the path.