To drop pounds this winter, try paying for groceries with cash instead of plastic. Research suggests shoppers who use real money to buy food tend to buy less junk than those who simply sign for it.  A recent grocery study set out to look for and measure the link between unhealthy food purchases and payment methods. When they looked at the data it showed that shoppers who paid with plastic spent far more and bought more junk and impulse items than customers who paid cash. SmartMoney reports recently that shoppers are showing a renewed preference for cash. If the trend continues, expect fatter savings accounts and slimmer waistlines.