One can pray with so much kavanah that after two or three words you would surely die, if not for HaShem's abundant kindness in maintaining you.Ok, so we can't necesarily start there so how do we get there? The Baal Shem Tov also says (in Tzava'ath haRivash) that one should start with as much kavanah as they can muster, and when they're all out of energy, finish with whatever energy HaShem gives us to continue.
(Baal Shem Tov, Tzava'at haRivash)
How have I implemented it in my life? focus intently on the beginning of Shema and the beginning of the Amidah (something we should be doing anyways) with all I have, then finish up by rote. (the way most of us do the whole davening anyways) To be on the safe side, in case the BeShT meant the very beginning of the Tefillah, I try to really put in kavannah in birchot
haShachar also; after that I more or less coast till Shema.