Prayer for oneself seems to work, optimism seems to work. Transcendental Meditation seems to work too.
But intercessary prayer for distant others appears ineffective, and can even worsen outcomes if the "beneficiary" of your prayers is notified.
Benson, Herbert, et al. "Study of the Therapeutic Effects of Intercessory Prayer (STEP) in cardiac bypass patients: a multicenter randomized trial of uncertainty and certainty of receiving intercessory prayer." American heart journal 151.4 (2006): 934-942.