What is the meaning of "Shisah Hashem Shamrah l'Fi"?
Rashi: When I pray to You, guard my mouth; I should speak clearly, that it will be accepted.
Radak: Shamrah is a noun. Its grammatical form is like Chachmah, Atzmah. Put a guard for my mouth, lest I stumble in my Tefilah. Rather, like my heart is proper in front of You, so my prayer should be proper - "l'Adam Ma'archei Lev ume'Hashem Ma'anei Lashon" (Mishlei 16:1).
Malbim (3-4): Put a guard on my mouth, that when my mouth will want to request something, the guard will stand to fulfill it immediately. The request is that my heart not veer.
What is "Dal Sefasai"?
Rashi: It is the upper lip (like Dalsos Sefasai). Radak - this is the opening to my lips, i.e. the key to my lips. Lips are called a door, because they open and close. It is proper to make one's lips like a door, which is opened and closed only for a need; so one should open his lips only for a need.
Radak citing his father: This is [lifting,] like "Dalu Einai la'Marom" (Yeshayah 38:14); it refers to the words that I will put on my lips.
Why does it say "Nitzerah Al Dal Sefasai"?
Radak: The matter is repeated in different words.
Malbim: Put a guard on the lifting of my lips. If I will lift my lips, even before I speak with my mouth, he will fulfill the matter. Notzer is a greater [guard] than Shomer.
