Perhaps the best advice is to use WERE for the present and WAS for the past. One great grammarian gives these examples: "She speaks as if she were ashamed" but "She spoke as if she was ashamed." One book used by many American college students simply gives this example: He acted as if he were/was losing his temper. Nowdays, many native speakers use WAS even for the present, e.g., She speaks as if she was ashamed.