In Jesus' Name, Amen

For the Son of God, Jesus Christ, whom we proclaimed among you, Silvanus and Timothy and I, was not Yes and No, but in him it is always Yes. For all the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory.
2 Corinthians 1:19-20

Prayer is drawing on the account where God has deposited all his promises of future grace. Prayer is not hoping in the dark that there might be a God of good intentions out there. Prayer goes to the bank every day and draws on promises of future grace needed for that day.

...

Prayer is the confident plea for God to make good on his promises of future grace for Christ's sake. Prayer links our faith in future grace with the foundation of it all, Jesus Christ.

Which leads to the final point: "Amen" is a full and precious word in times of prayer. It doesn't mean primarily, "Yes, I have now said all this prayer." It means primarily, "Yes, God has made all these promises." Amen means, "Yes, Lord, you can do it." It means, "Yes, Lord, you are powerful. Yes, Lord, you are wise. Yes, Lord, you are merciful. Yes, Lord, all future grace comes from you and has been confirmed in Christ." "Amen" is an exclamation point of hope after a prayer for help.

When we come to the end of our prayers and say the simple words, "In Jesus' name, Amen," we are really saying two Amen's. When we say, "In Jesus' name," that is God's Amen to us. All his promises are Amen in Jesus: Jesus Christ is God's Yes and Amen at the end of our prayers. Then when we say, "Amen," this is our Yes and Amen back to God for his. Which means that our Amen, and the prayer it supports, is our Yes to God's Yes to us. It is a commitment from our hearts that we will now live by faith in the Yes of God's guaranteed future grace.

--John Piper, Future Grace, p.107-108

Amazing. Paul (inspired by the Holy Spirit) said it himself: in Jesus, God always says yes to us. He never says no...as long as we're asking for what He's promised in His Word.