Jesus uses glory as the means to show He existed before His incarnation:
1 When Jesus had spoken these words, he lifted up his eyes to heaven, and said, “Father, the hour has come; glorify your Son that the Son may glorify you, 2 since you have given him authority over all flesh, to give eternal life to all whom you have given him. 3 And this is eternal life, that they know you, the only true God, and Jesus Christ whom you have sent. 4 I glorified you on earth, having accomplished the work that you gave me to do. 5 And now, Father, glorify me in your own presence with the glory that I had with you before the world existed. (John 17) [ESV]
First, since the Father sent Him, Jesus had to be in existence before He arrived. Second, Jesus requests to be glorified in His Father's presence with the glory He had [in His Father's presence] before the world existed. This is something which He had with the Father before being sent. The Greek is εἶχον, the imperfect tense of ἔχω "to possess." This displays an internal aspect. "That is, it portrays action from within the event, without regard for beginning or end."
This leads to two conclusions. First, Jesus could not have been created. For if He had been created, He would need to ask to be restored to the glory He had received from the Father. Second, the glory is that which the Father [still] possesses, and so this glory, like the Father, must have no beginning. Therefore, this is not only a statement of existence before being sent; it is made in terms which requires equality of pre-existence with the Father.
Based upon the Old Testament, Jesus' request is consistent with His deity:
8 I am the LORD; that is my name; my glory I give to no other, nor my praise to carved idols. 9 Behold, the former things have come to pass, and new things I now declare; before they spring forth I tell you of them.” (Isaiah 42)
Jesus is not getting the Father's glory: He is sharing in it. Moreover, the "new thing" was to glorify the Father on earth.
"Glory" also points back to the Prologue:
And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. (John 1:1)
Therefore, when the Father answered Jesus' prayer and showed the disciples this glory after the Crucifixion, the prophecy in Isaiah was fulfilled:
And the glory of the LORD shall be revealed,
and all flesh shall see it together,
for the mouth of the LORD has spoken.” (Isaiah 40:5)
1. Daniel B. Wallace, Greek Grammar: Beyond the Basics An Exegetical Syntax of the New Testament, Zondervan, 1996, p. 541