This is from Eric Barker, who does the fantastic Barking Up the Wrong Tree blog. There are a lot of excellent tips here for improving performance.
The most important tip? Arousal control through mental preparation:
This is why Musashi and most martial arts practitioners focus on mental training as much as on physical training. Both are equally important — and require equally vigorous exercise and practice.
Another good one? Monitor your breathing:
Teaching recruits to monitor their breathing helped increase Navy SEAL passing rates from 25 to 33 percent.