I used to believe that you could change the culture or behavior of a company. I still believe it's possible, but it is at least a five to ten year process, if you are successful at all. More recently, I have been attracted to the ideas of the behavioralist, Edgar Schein. Schein has argued that you cannot change the culture of a company, but you can use the culture of a company to create change. It's an interesting approach to overcoming resistance. And if you can change how a company does its work, you might eventually be able to change how its people think.
The thing that I have been emphasizing in my own work for the last few years has been the group approach. To try to buy groups of stocks that meet some simple criterion for being undervalued-regardless of the industry and with very little attention to the individual company.