The Mishna Berura (185:1) cites the Sefer Ha’hinuch’s remark that one who ensures to recite Birkat Ha’mazon properly will always be provided with a respectable livelihood, throughout his life.
In explaining the Sefer Ha’hinuch’s comment, the Mishna Berura writes that this means, for one thing, making a point to recite Birkat Ha’mazon from a written text, and not from memory. Indeed, Hacham Baruch Ben-Haim would always tell us, "Otiyot Mahkimot" – "Letters make one wise." When we see the words, we are better able to concentrate on and understand their meaning. Moreover, we are less prone to forgetting the special additions for Shabbat and holidays if we recite Birkat Ha’mazon from a written text.
It is worth mentioning in this context a remarkable story told of Hacham Ovadia Yosef when he was a six-year-old student in Rabbi Natan Saleem’s Yeshivat Beneh Sion in Jerusalem. The school did not have printed Birkat Ha’mazon cards like we have today, and so Hacham Ovadia sat down and wrote cards for the children in his class, so they could all recite Birkat Ha’mazon from a written text. Already at this young age, the Hacham understood the importance of reciting Birkat Ha’mazon from a text, instead of reciting it from memory.
Additionally, one should recite Birkat Ha’mazon in an audible voice, such that he can hear the words he recites. It goes without saying that one does not fulfill the obligation if he just reads the words with his eyes and does not recite them with his mouth. But in addition, one should recite the words in an audible voice. The Shulhan Aruch rules that after the fact, if one did not recite Birkat Ha’mazon in an audible voice, he has nevertheless fulfilled the obligation. Interestingly, however, the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that since some authorities ruled that one does not fulfill the obligation if he recites Birkat Ha’mazon inaudibly, one who did so should then eat more bread and recite Birkat Ha’mazon properly in order to satisfy all opinions. Halacha does not follow this view, as Hacham Ovadia writes, but the Ben Ish Hai’s ruling demonstrates to us the importance of reciting Birkat Ha’mazon in an audible voice, and not silently.
And, of course, one must try to concentrate on the words of Birkat Ha’mazon as he recites them.
The Bah (Rav Yoel Sirkis, 1561-1640) observes that the Peh Sofit does not appear anywhere throughout the text of Birkat Ha’mazon. The reason, he explains, is because this letter is associated with words that refer to calamity (such as "Shesef," "Kesef," and "Af"), and reciting Birkat Ha’mazon properly protects one from harm. It is well worth our while, then, to recite Birkat Ha’mazon slowly and patiently, and with concentration, which takes just a few minutes, fulfills (in many situations) a Torah obligation, and brings us great benefits both in this world and in the next.
Summary: It is preferable to recite Birkat Ha’mazon from a written text, rather than from memory. Halacha requires reciting Birkat Ha’mazon in an audible voice, rather than silently, though after the fact, one who recited it silently has fulfilled his obligation (as long as he actually said the words with his mouth, and not just with his eyes). The merit of reciting Birkat Ha’mazon properly brings protection from harm as well as material blessing.