Mental toughness...

While the purpose of this article seems to focus on selling a product to golfers, it does focus on something you must have to play a winning game of darts...

Mental toughness in golf is developed and built over time. Once you have a degree of mental toughness, access to it when it matters becomes easier and easier. Like any new skill it habituates over time. Your goal is to have an inner resolve that generates consistent, focused swings.
(full article...)

A Dart site that is at least worth a look...

DartPractice.com has been created to help (dart) players keep track of their steel tip and soft tip practice scores to see how they are improving over time.
Check it out at the following url: http://www.dartpractice.com/