What is the ruling on repeating the same supplication formula in every prayer, and what is the best way to begin a supplication?

Prostration is a position where the supplication is hoped to be answered, and it is sunnah (definition) to make a lot of supplications during it, as the Prophet, peace and blessings be upon him, said: "As for prostration, strive in supplication, for it is likelier that you will be answered." It is permissible to supplicate with non-narrated supplications, but one should not adhere to a specific supplication and be constant in it, as if it were a narrated sunnah; because that might fall under additive bid'ah (innovation).