Character

You might have heard this line somewhere: value investing is a relatively simple concept but it is also the toughest to execute. Many pointed out that having a contrarian mindset is necessary, and I do feel this is only partially true.

Warren Buffett once said that "the idea of buying dollar bills for 40 cents take immediately with people or it doesn't take at all. It is like an inoculation. If it doesn't grab a person right away, i find that you can talk to him for years and show him records, and it doesn't make any difference. They just don't seem able to grasp the concept, simple as it is." (The Super-Investors of Graham-and-Doddsville)

I think this is largely due to a person's character which is either genetically inherited or moulded by the environment... and no amount of schooling might be sufficient to change one's attitude.

1) Healthy Respect for Money
Money is much easier to lose than keep. This is the main reason why value investors are usually natural savers, and why we tend to look down (at the potential risks) than up (rewards, or price up-side).

2) Objectivity and an Open Mind
You should be willing to assess any investment objectively. The company that is having tons of trouble in the news could be the next perfect opportunity. If you have problems discussing the strengths of your enemies among your peers, you might not be suitable to invest with a value approach.

3) Self-confidence, not Arrogance
Arrogance is believing that you are the best and are right at all times. Self confidence is having done the work, believing that you are right but having a healthy amount of self-doubt to check and re-check.

4) Toughness
Be ready to hold on to your beliefs even if the market adjust the price of your investment by 20-40%. One could argue that this mindset can deepen your losses (Bill Ackman's Herbalife short positions and Bruce Berkowitz's Sears comes to mind)

5) Be really optimistic; Bad times don't last
Pretty self-explanatory.