BROOKLYN - Healthy competition has long been known to drive business, but this month, stores along Washington Avenue have decided to team up to help each other boost their holiday sales. As part of the initiative organized by the Washington Avenue Prospect Heights Association, customers who spend money at one of the 19 participating businesses will get a discount at another."In that way, we tend to help each other promote each other's businesses," says hairdresser Avion Julian. "It's like keeping the business within the family." Anyone who spends $50 or more along Washington Avenue will get a $5 certificate that could be used to shop in the area through Jan. 30. Local business owners say they haven't increased their prices to make up for the discounts.