From this New York Magazine article Why Running Helps Clear Your Mind -- Science of Us:
Studies in animal models have shown that new neurons are produced in the brain throughout the lifespan, and, so far, only one activity is known to trigger the birth of those new neurons: vigorous aerobic exercise, said Karen Postal, president of the American Academy of Clinical Neuropsychology. “That’s it,” she said. “That’s the only trigger that we know about.” Vigorous exercise in the form of High Intensity Interval Training (HIIT) is what we do at New Orleans Fitness Trainers and Austin Personal Training. Regular exercise improves our ability to think and remember through the creation of new brain cells, a process known as neurogenesis, the growth and development of nervous tissue. Another blog post on the same subject: Growing new brain cells by exercising.