While your student will likely need a backpack, lunchbox and crayons by the first day of school, items such as sweaters and corduroy pants can wait until at least a few weeks into the semester. Fall apparel items are priced highest at back-to-school time, and retailers will usually discount them by mid-September at the latest, says Nelson. If your child wants a new outfit for his or her first week back, buy a few clearance-priced summer items.

"My kids are going back to school in August and they don't need fall clothes yet," Nelson says. "I think we tend to get caught up in buying clothes when retailers tell us to, not when the weather dictates it."

You can also cut back-to-school costs by scouring office-supply stores after the back-to-school rush is over, says Demer. Most retailers will put leftover school supplies on clearance, enabling you to scoop up supplies for next year at up to 90 percent off.

"Paper, crayons, folders and glue sticks always seem to be in high demand when children are in the house, so stock up now," Demer says.