What is "Kol Mishmar"?
Rashi: It is everything that the Torah warned. Guard your heart from transgressing it, whether it is a light or severe Aveirah.
R. Yonah: Every bad Midah.
How does one guard his heart?
R. Yonah: Do not think about cruelty, anger or stinginess. Via the heart, you will eradicate all bad Midos. You will refrain from thinking about them, and think about their detriment, which is great, and long for the opposite of them.
Malbim: It says "Heitivah Hashem la'Tovim veli'Ysharim b'Libosam; veha'Matim Akalkalosam Yolichem Hashem Es Po'alei Aven" (Tehilim 125:4-5). I explained that it is primary to be straight in the heart and conscience, even if sometimes his revealed deeds are not straight. Ikul is something that is internally straight, but appears crooked externally. As long as they are straight and good in their hearts, Hashem will do good to them. One who tilts them to consider them liable due to the external crookedness, Hashem will punish him for this, for 'Hashem desires the heart' (Sanhedrin 106b). From everything from which you must beware, it is primary to guard the heart from that evil.
What is the significance of "Ki Mimenu Totz'os Chayim"?
Rashi citing Tanchuma: From what is easy, you will have life and great reward if you fulfill it.
Malbim: The primary source of life is the heart. It is primary; the body is secondary to it. The external deed is not evil if his conscience is straight - an Aveirah Lishmah is greater [than a Mitzvah Lo Lishmah - Nazir 23b].
