There's a nice article on developing mental toughness on Ralph Jean-Paul's blog. The main key to developing mental toughness is the same as developing a muscle: repetitive use and practice.
Interestingly, the development of mental toughness is also a spiritual battle (Biblically) (see the last paragraph where Paul speaks of beating his body into subjection), and while normally I wouldn't throw a religious link into the mix here, I think it's important to recognize, on some level, that in order to successfully develop a better, stronger attitude, the spiritual side is certainly an issue.
I would consider looking into most of what are known as the "Spiritual Disciplines" as a method for developing mental toughness. These include fasting, meditation, prayer, servitude, simplicity, etc. All of them will have a positive effect both on you, and on those around you, and will do wonders at developing your character and inner strength.
Note: I own the blog referencing Paul's spiritual battle. However, this same information can be found at many other sources through a quick search on Google.
Addressing comments about the spiritual aspects of mental toughness, take a look at this quote from the Fort Hood Resiliency Campus of the US Army's web site:
Any human being functions in three modes i.e. Physical (Body), Mental (Mind), and Spiritual (Soul). These three aspects of our lives are not independent from each other, but dependent and usually intertwined. They are reflected in our behavior, our relationship to others, and also seen in the way we respond to challenges and crisis.
The Stoic philosophy is another spiritual discipline that leads to mental toughness, and that is not a religion, which you might want to look at. One excellent source of information (links, on-line books, videos, training courses, etc) is the Stoic Foundation.