Ms. Shoshana Averbach, LMSW (among other things), forwarded a mention of the following story to me:
There was once a chassid whose son was very ill. After a prolonged illness, the physicians finally told him that there was no hope. There was nothing more they could do; they did not know if the child would live.
The chassid was devastated. He hurried to Lubavitch and approached the Tzemach Tzedek, the third Lubavitcher Rebbe. Overcome with grief, he could barely mouth his request for a blessing.
The Rebbe answered him briefly in Yiddish: טראכט גוט וועט זיין גוט (Tracht gut vet zein gut); “Think positively, and the outcome will be good.”  [lit.: “Think good and it’ll be good”]
As the chassid walked out of the Rebbe’s room, he pulled himself together. He put himself in a state of mind that radiated utter confidence. He knew G-d could help him and cure his son. And he believed that this would happen.
When he came home, he was told that there had been a sudden change in his son’s condition. The physicians had no explanation, but the child had definitely taken a turn for the better. When the chassid inquired, he was told that the change took place at exactly the time that he visited the Rebbe.
[Rabbi Shlomo Majesky comments]
The story shows us that thinking positively produces two effects:
a) when a person is in high spirits, he functions better; and
b) thinking positively itself brings about positive change. By envisioning good in one’s mind, one creates positive spiritual influence that enables that picture to materialize.
(“The Chassidic Approach to Joy” by Rabbi Shlomo Majesky; ch. 12; on Chabad.org)
 See Sefer HaSichos 5687, p. 113 and sources cited there; explained in Likkutei Sichos, Parshas Shemos 5751