Tefilin and sichat chulin

“And this shall serve you as a sign on your hand and as a reminder on your forehead —in order that the Teaching of יהוה may be in your mouth—that with a mighty hand יהוה freed you from Egypt.

The holiness associated with tefillin is very great. As long as a person is wearing tefillin on his head and arm, he will be humble and God-fearing and will not be drawn to frivolous behavior or empty speech. He will not turn his thoughts to evil matters, but rather will direct his heart to words of truth and justice.
Accordingly, a person should try to wear [tefillin] throughout the entire day, for this is the mitzvah associated with them. Among the praises conveyed upon Rav, the student of Rabbenu Hakadosh, was that he was never seen walking four cubits without [reciting words of] Torah, without tzitzit, and without tefillin.

Abaye said to him: It is permissible for me because I am donning phylacteries now and as long as they are upon me they ensure that the fear of God is upon me.

One should have in mind, when putting them on, that God commanded us to place these four passages - which contain the Unity of His Name and the Exodus - on the arm opposite the heart and the head opposite the brain, so that we may remember the miracles and wonders that He did for us which indicate His Unity and that He is omnipotent in heaven and on earth to do with them as He wills. And [also in order] to subjugate, to God, one's soul which resides in the brain, as well as his heart which houses one's main physical desires and thoughts. Through this he will remember his Creator and moderate his pleasure[-seeking]. He should put on the arm-tefillin first and recite the blessing "... to place tefillin" and afterwards put on the head tefillin. He should only recite [that] one blessing for both of them. Rem"a: Some say to recite [the following] blessing on the head tefillin "...on the mitzvah of tefillin" even if one did not make an interruption between them (ie. the arm and head tefillin) [The Ro"sh in the Laws of Tefillin]. [This is the widespread Ashkenazic custom, to make 2 blessings]. It is good to always recite after the second blessing: "Blessed is the Name of His glorious Kingdom forever and ever" [Mahar"i ben Chabib Agur, Siman 38]

One should have in mind, when putting them on - The Ba"ch, in siman 8, wrote a reason for this - since it says (Exodus 13:9) "And it shall be to you as a sign on your arm ... so that Hashem's teachings will be in your mouth, for with a strong hand Hashem brought you out...", it teaches us that the main mitzvah and fulfillment of it is dependent on one's thoughts - that one should have this in mind at the time he is fulfilling the mitzvah. Pri Megadim wrote that, nevertheless, after the fact, even if one only had in mind that he is [putting on his tefillin] for the sake of the mitzvah, he has fulfilled his obligation.