A few years ago, I met a rav. He said, "You know I am not the type to run after segulos. It isn't my way. But, twenty years ago, I didn't have children, and that was after twenty-two years of marriage. Someone put into my mailbox an article that discusses the segulah of saying Tehillim twice on the night of Rosh Hashanah. I did this segulah, and nine and a half months later, I had my first and only child." He added that his daughter is a kallah.
He also told me that he shared this segulah with a class of thirty older bachurim. On Chanukah, he received a phone call from one of the bachurim, who said he was the th bachur of the grade to get engaged.
Menachem Holzberg from Miami didn't have children for twenty years. He told me he heard about the segulah and said Tehillim twice on Rosh Hashanah night. (When he felt tired, he went outside to wake up.) Nine months later, he made a bris. He says that they went to many doctors, but nothing helped. But twice Tehillim on Rosh Hashanah night was his salvation. The child was born naturally, just with Hashem's blessings.
One Rosh Hashanah, a yungerman from Bnei Brak got up at 2:00 am and said twice Tehillim. His wife often required hospitalization, but that year, she didn't need to be hospitalized.
We don't know how these things work and why specifically Tehillim twice, but I found an excellent source from the Dubno Magid zt'l.
The Dubno Magid tells a mashal of a Yid who lived among goyim, and the goyim caused him much suffering. When the king visited his city, he wanted to tell the king what he was going through, but the goyim were also there, before the king, and he knew that they would convince the king to ignore his accusations.
The Yid sought counsel from a friend. The friend said, "The goyim are present during the daytime. They aren't with the king at nighttime. Go to the king at night and tell him what you are going through."
The Yid followed this counsel, and the king helped him.
The Dubno Magid says that the same is true with Rosh Hashanah. Rosh Hashanah, by day, there is a great judgment, and mikatrigim speak against us. But the court of heaven doesn’t begin at nighttime, and the mikatrigim aren't present. Now is the time to plead before Hakadosh Baruch Hu, and your tefillos will be answered. The Dubno Magid writes, quoting from the kadmonim (gedolim of generations ago), that it is therefore good to daven Rosh Hashanah at night before the mikatrigim arrive.
The Dubno Magid writes that this is alluded at in the pasuk (Eichah 2:19) לבך כמים שפכי ,אשמורות לראש בלילה רני קומי 'ה פני נכח, "Arise, cry out in the night, at the beginning of the watches! Pour out your heart like water before the presence of Hashem." 'ה פני נכח means when you are alone with Hashem. That is an excellent time to pour out your heart before Hashem, and your tefillos will be answered.
Here are some more stories of salvations that occurred in the merit of saying Tehillim twice on Rosh Hashanah night.
One Erev Rosh Hashanah, the night of the selichos (called Zechor Bris), I received a call from a famous lawyer in Eretz Yisrael. I was still in the car, in front of the beis medresh "Dushinsky", where I was scheduled to deliver a drashah. I told him that I didn't have time to talk because I must give the drashah, but he insisted it would take only a minute.
He said that his daughter is forty years old. They waited all this time for her to become a kalah, and now they were moments before "breaking the dish", to finalize the shidduch that just occurred. He told me that exactly a year before, he did the segulah of saying Tehillim twice on Rosh Hashanah evening, and he had his salvation... (When I came to the beis medresh to give the drashah, I began the drashah with this story).
A yungerman from Beitar had four children who were all in shidduchim, ages 23-28. He wanted to say sefer Tehillim twice on Rosh Hashanah night, but after the Rosh Hashanah seudah, he realized that it wasn’t going to happen. So, he divided the Tehillim among his family. Each member would say about fifty chapters. They said the Tehillim with hislahavus for about forty minutes, and in this way, they finished the entire sefer Tehillim more than two times.
All four children got engaged and were married that year!
Year 5780, a yungerman from Montreal wrote me, "I have eight children, four of them are married, and I have nachas from them. But the last chasunah I made was in the year 5773. Now I have two bachurim. One is thirty and one is twenty-seven, and I have two girls, ages twenty-five and twenty-two. I did the segulah together with my children. Each of us took some chapters to say, and collectively, we finished Tehillim twice. That year, three of my children got engaged. I plan to say Tehillim this year as well, and hopefully, my twenty-seven-year-old son will also become a chasan."
He added in the letter, "I am not a wealthy person, but I was able to marry them off without borrowing a penny."