Elevate the head of your entire bed 4-6 inches to achieve a 20 degrees or more slant.
For quite some time research has recommended left side sleeping as the best positioning for help with acid reflux. In one study in The Journal of Clinical Gastroenterology scientists took healthy subjects, fed them high fat meals to induce heartburn, and immediately after the meals had the subjects lie on either their left or right side while devices measured esophageal acidity. They found reflux time significantly greater when subjects were on their right side. And, "average overall acid clearance was significantly prolonged with right side down".
Another study published in The American Journal of Gastroenterology, showed the same result - right side sleeping tends to aggravate heartburn.
Both of these studies were done on a relatively small number of people.
Stomach sleeping is never recommended for people with acid reflux, sleep apnea or any other breathing or sleep related problem. The reasoning is that you do not want to put extra pressure on your stomach.
Good luck,
Kitt