In the second part of "To the Memory of My Beloved Master William Shakespeare," Johnson uses the rhetorical device of negation in his heroic couplets in order to praise Shakespeare:  "He was not of an age, but for all time!" (line 43) and

Yet must I not give Nature all [the credi]; thy art,/My gentle Shakspeare, must enjoy a part (lines55-56)

This use of negation increases the superlative qualities of Shakespeare's achievement.  In the first part, Johnson does the same as diminishes the credit of others to enlarge Shakespeare:

...I will not lodge thee/ by Chaucer or Spenser, or bid Beaumont lie/A little further to make thee a room:/Thou art a monument without a tomb. (19-23)

In addition to this technique of negation, Johnson uses the metaphor of lineage to praise Shakespeare as he has made Johnson and others heirs through the universality of his genius.  Therefore, they owe their greatness in part to Shakespeare:

...Look how the father's face/Lives in his issue, even so the race/Of Shakspeare's mind and manners brightly shines/In his well torned and true filed lines;/In each of which he seems to shake a lance,/As brandisht at the eyes of ignorance./Sweet Swan of Anon!  what a sight it were/To see thee in our waters yet appear (65-71)

Succeeding generations of writers have been inspired by Shakespeare's line to emulate his genius and give enjoyment to people and "shake a lance" at lack of knowledge.  Shakespeare remains the "Star of Poets" for all to look up to and admire.