Because tefillin need a clean body, the minhag has become not to wear them all day long. Nevertheless, every man must have them on while saying Shema and tefilla. (ShA2)
Great is the reward for tefillin. Including a guarantee for Olam Haboh – protection from the fires of Gehinnom – forgiveness from all sins. (ShA 1 – MB1)
At the same time, if out of disgrace a person doesn’t wear them, he is in the category of Poshay Yisroel B’gufon.This is rebuke for someone who is indifferent about placing the tefillin on the correct place (e.g. the shel rosh down on the forehead) – it is as if he didn’t wear tefillin at all. (ShA1 – MB 3)
In light of the above, every person should buy tefillin from an expert sofer, a yoray Shomayim and Baal Torah.So too, the straps – buy them from a trustworthy individual who knows to make them lishman.If the tefillin are possulin the berachos are also l’vattala.You spend a lot on beautiful clothes and furnishings, certainly you should not skimp on cheftzay Shomayim. Pay alot to make sure that your tefillin are kasher. (MB 4)
Beautify the tefillin by means of nice klaff, nice ink, a nice quill and an expert sofer. (MB 4)
If for some reason you didn’t have tefillin then, you are still obligated all day long not to let the day pass without having layed them. (MB 6)
Men of great deeds learn Torah after tefilla with their tefillin on. They must beware of idle speech which also removes their minds from their tefillin. (MB7)
Nowadays, the minhag is to train boys to lay tefillin and learn the relevant laws two or three months before turning thirteen years old. (MB12)