"that I command you this day": They should not be to you as an antiquated edict, which no one takes to heart, but like a new one, which all run to read.
"upon your heart": From here R. Yoshiyah was wont to adduce: A man should beswear his (evil) inclination (not to do what is forbidden.) For thus do we find with all the righteous ones, that they beswore their inclinations:
Abraham — (Bereshith 14:22-23) "I lift my hand (in oath) to the L-rd, G-d on high, Possessor of heaven and earth: Nothing (shall I take), from a thread until a sandal latchet, nor shall I take anything that is yours." Boaz (to Ruth) — (Ruth 3:13) (I swear) "As the L-rd lives, lie (here with me, unmolested) until the morning…" David — (I Samuel 26:10) "And David said (to Avishai, who wanted to kill Saul, David's enemy): (I swear) as the L-rd lives: The L-rd will strike him with plague, or his day will come and he will die, or he will go forth in battle and he will perish." Elisha — (II Kings 5:16) "(I swear) as the L-rd lives, before whom I have stood, I shall not take (any emolument, etc.") And just as the righteous beswear their (evil) inclination not to do (as it wishes), so, the wicked (beswear it to do) as it wishes, viz. (Ibid. 20) "As the L-rd lives, I (Gechazi, Elisha's servant) shall run after him and take something from him!"