One scanning technique that was taught to me by a cousin who used to fly F-4's for the Navy during my PPL training works very well and has served me thru the years. If you have VFR flight following services (always nice to have,but DON'T be overly reliant on it!),i.e. "traffic 5 miles...." first focus out at 5 miles distance and THEN look for the target. This method helps to alleviate the empty field myopia affect mentioned in an earlier post.