A man once approached the Rov of Yerushalaim, Rav Yosef Chaim Sonnenfeld zy”a, and told him that he was having problems with parnassah. Rav Yosef Chaim told him that he should concentrate when he recites Birchas Hamazon, as the Sefer Hachinuch (Mitzvah 430) writes: “I have received a tradition from my teachers that anyone who is careful with Birchas Hamazon will be supported honorably throughout his life.”
The man said that he had tried this and it hadn’t helped, and the Rov told him, “Do this with hasmadah. Keep it up and don’t stop and, in the end, you will see your salvation.”