Who will believe my verse in time to come

If it were filled with your most high deserts?

Though yet heav'n knows it is but as a tomb

Which hides your life, and shows not half your parts.      (4)

If I could write the beauty of your eyes,

And in fresh numbers number all your graces,

The age to come would say, "This poet lies-- 

Such heav'nly touches ne'er touched earthly faces."          (8)

So should my papers, yellowed with their age,

Be scorned, like old men of less truth than tongue,

And your true rights be termed a poets rage,

And stretchéd meter of an Antique song.                            (12)

But were some child of yours alive that time,

You should live twice in it, and in my rhyme.

(1-2) The first two lines, at a glance, can be read as heading towards an insult: Who would think me an honest person if I wrote great things about you? From this starting position, the lines that come next to clarify the context are that much sweeter. This is classic obsession also—have you ever hear someone talk about someone they're obsessed with, they'll fill pages of how beautiful and perfect the person is. He's saying I know you'll think that, so I'm not even going to bother, no one would believe me.

(3-4) The writer says his poem is "but a tomb" for the boy, because while it preserves, it also "hides" his life due to the impossibility of describing his beauty or of showing even half the facets of who he was (his "parts"). The sonnets also hide the boy's life in the sense that they don't disclose his identity.

(5) The poet doesn't describe the boy's eyes, probably because "writing beauty" can hardly ever avoid cliches. The wording also suggests that any description would be inadequate: "If I could write the beauty of your eyes." 

(6) If he were to count all the boy's graces he would need "fresh" numbers in order to do it: that is, numbers that haven't yet been discovered/invented. His reluctance to cheapen love with calculation or description reminds me of an exchange in Antony and Cleopatra:

Cleo: If it be love indeed, tell me how much.

Ant: There's beggary in the love that can be reckon'd.

Cleo: I'll set a bourn how far to be beloved.

Ant: Then must thou needs find out new heaven, new earth.

Antony won't stoop to describe "how much" he loves, but refuses quantify something that to him is unquantifiable. To him, it cheapens it.

(7-12) Again we find the poet looking ahead to what future generations will think. If he were to describe all he thinks is beautiful in the boy future generations will think him a madman. They would chalk it up to a poet's "rage": a mad, passionate love. But the poet doesn't want to be seen as a man stretching the truth; an old fool in love. He wants to be seen as someone who is, if anything, withholding. Not exaggerating.

The final couplet returns the idea of the boy living on after death. If the boy has a child, his beauty can have two more lives - both in the child, and in the poem.

From fairest creatures we desire increase,

That thereby beauty's rose might never die,

But as the riper should by time decease,

His tender heir might bear his memory:                   (4)

But thou contracted to thine own bright eyes,

Feed'st thy light's flame with self-substantial fuel,

Making a famine where abundance lies,

Thy self thy foe, to thy sweet self too cruel:             (8)

Thou that art now the world's fresh ornament,

And only herald to the gaudy spring,

Within thine own bud buriest thy content,

And, tender churl, mak'st waste in niggarding:     (12)

Pity the world, or else this glutton be,

To eat the world's due, by the grave and thee.

Who will believe my verse in time to come

If it were filled with your most high deserts?

Though yet heav'n knows it is but as a tomb

Which hides your life, and shows not half your parts.      (4)

If I could write the beauty of your eyes,

And in fresh numbers number all your graces,

The age to come would say, "This poet lies-- 

Such heav'nly touches ne'er touched earthly faces."          (8)

So should my papers, yellowed with their age,

Be scorned, like old men of less truth than tongue,

And your true rights be termed a poets rage,

And stretchéd meter of an Antique song.                            (12)

But were some child of yours alive that time,

You should live twice in it, and in my rhyme.