Note: See below for instructions on how to fulfill the mitzva of separating challah - the most important part of baking challah. It is said in the name of Rav Dovid of Tolna that one should make sure the dough separated as challah is burnt completely until no part of it is edible. In addition to being the ideal way to burn the challah, he says that if a davar tamei eats the unburnt challah, it can cause one’s children to go off the derech, chas v'shalom.
The segulah that I received from my Rebbe, the Clevelander Rebbe Shlit"a of Ra'anana, is that on the first Shabbos after Pesach you "knead the challah dough" with a key. In fact, that is the custom recorded and found in the sefer Ohev Yisrael of the Apta Rebbe. Our custom is (I wash the key first in soap and hot water) to knead the challah dough and poke the key into the dough while reciting the open the shaarim prayers found in most zemiros bentchers at the end of the zemiros for Melave Malka - where we ask Hashem to open many gates for us. Other people I have seen also bake a key shape on the challah, but I didn’t receive any such tradition.
The Ohev Yisrael says:
לשבת שאחר הפסח המנהג הוא מימים קדמונים לנקוב את החלות בשבת שלאחר פסח במפתחות, ונעשה .על החלה צורת מפתח
In at least three other places, the Ohev Yisrael uses similar wording to ונעשה על החלה צורת מפתח. This is probably where the minhag of some to bake a key shape image on the challah comes from.
