Nike Dunks, like all sneakers, are designed to be supportive and comfortable. Unlike dress shoes and boots, sneakers are worn as much for comfort as for fashion and function. However, while sneakers usually won't require the same painful breaking-in period that leather dress shoes and boots require, you may still want to loosen your Dunks if they feel a little stiff and tight.
Dab some rubbing alcohol onto a cotton cloth. Rub the inside of your Nike Dunks where they feel too tight on your feet. The alcohol will soften the leather.
Rub a damp cotton cloth on a bar of saddle soap until it develops a lather. Apply the saddle soap to the outside surface of your Dunks, covering the entire surface of the shoes. Saddle soap will soften leather while moisturizing and keeping it in good shape.
Wear an extra pair of socks and put on your Dunks. Tie the shoes tightly and wear them around the house during the day. Remove them when your feet begin to hurt. Do this for a couple of days until your shoes soften and loosen up.