This does not mean slouching your shoulders, but think about ‘pulling your shoulders away from your ears.’ Keep your shoulder blades to neutral, not too tensed or not too slouched down. This helps your arm swing and trunk rotation when you run.
Don’t put them inside your pockets, but when you swing your arms keep them to the level where your pockets are or where your hands might slightly brush your hip. Keep your elbow at a 90 degree angle and point your hands forward. Remember that you want to move forward so you should swing your arms forward as well, not bringing your hands in front of your chest to keep your trunk from twisting too much.
You can observe that a lot of runners tend to clench their fists. This small movement causes tightness around the arms and even tension on the neck and shoulders. Imagine holding eggs with each of your hands – you don’t want to break them so gently cup your hands and relax.
I once went to a department store in the Philippines to look for a pair of shoes. One salesman ambushed me with his trivia to convince me to buy his shoes. He said that runners should ALWAYS land on their mid-foot. I disagree with this notion because runners have significant individual differences from one another. Most runners would be comfortable landing on their mid-foot but if you have a high arc, it’s a different story. Also in terms of technique, if you want to sprint then you’re better off landing on your toes. Some people will be more comfortable landing on their heels and then rolling their foot, as long as they have enough heel cushioning. What’s more important is your ‘stride length’ – you would not want your leg to be way ahead of you unless you are sprinting. Your feet should land directly underneath your body and as your foot strikes the ground, your knee should be slightly bent naturally upon impact.