Usually, U.S. retailers earn 20-80% of their entire yearly gross during the holiday season. But this year, things are different. The sluggish US economy has prompted Deloitte Research’s Carl Steidtmann to fear “the worst Christmas ever” in the retail sector. Short of going door to door in a Santa suit, how can businesses increase profits and prevent a blue Christmas? Despite economic gloom, industry leaders have found that an e-mail follow up strategy can increase sales by a cheery …