1) The Shulhan Aruch[36] gives the following description of the intention one must have while laying tefillin: 

“One should have mind while placing them that the Almighty commanded us to place these four sections [of the Torah] — which contain the oneness of His Name and the Exodus — on the arm, next to the heart, and upon the head, next to the brain, in order that we remember the miracles and wonders that He performed for us, which demonstrate His oneness and that He has the power and authority in both the upper and lower worlds to do with them as He wishes. And one must subjugate to the Almighty the soul, which is located in the brain, and also the heart, which is the primary [source] of desires and thoughts. One will thereby remember the Creator and minimize his [earthly] pleasures

The required intention when laying tefillin is thus the following: “God commanded me to tie to my arm and head the sections of the Torah that tell of the Exodus and His oneness, and which testify to His unlimited power, and I therefore commit my thoughts, desires and mind to the service of the Creator.”

2) It is proper to verbally declare this intention by reciting the “Le’Shem Yihud” prayer printed in the siddurim.[37]

If one did not have this intention when laying tefillin, he nevertheless fulfilled his obligation, so long as he had in mind to fulfill the mitzvah of tefillin.[38]