Backspin is a result of hitting down sharply on the ball. However, no matter how well you execute this, backspin also depends on your ball and club. A prov is going to spin better than a DT solo, and a wedge is going to spin more than a three iron. Also, take into consideration the course and weather. If the course is too firm of greens it is going to be tough to spin it no matter how much backspin is applied, (US OPEN), but that can all change with water in the form of rain or grounds crews (US OPEN again, ha ha). But just hit down more steeply and take a reasonable divot.