"And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown but we an incorruptible." (1 Corinthians 9:25)
On your laurels
"Be ye therefore perfect, even as your Father which is in Heaven is perfect." (Matthew 5:48)
How do we relate to this? A God that without Jesus, we cannot relate to. A God, who, without Jesus as interlocutor and savior we cannot look at and call Father. So when Jesus says "be perfect", we must get His idea of what perfection is. And then we go and make a mistake and then think perfection is unattainable. Or forget altogether about this integral part to the Sermon on the Mount.
Beginning from the bottom, it's a gift when the Lord shows us up. When He reveals to us the wide swaths to our character that are very unperfect. And then when we think, however incorrectly, that "zero defects" (the detectable ones, you understand) is what it means to "be perfect", I think we may've missed the boat. What do you think?
"But let patience have her perfect work, that ye may be perfect and entire, wanting nothing." (James 1:4)
On your mark
I think the connotation of "perfect"—because it's the same Greek word as when it was translated from Jesus—is more in keeping with the definition from James' letter. And not to downplay in the slightest what Jesus said. But when James writes "perfect", he's talking about becoming more and more who the Father made you to be.
"Thy hands have made me and fashioned me: give me understanding, that I may learn Thy commandments." (Psalm 119:73)
Did you know God loves you? God loves you whether you made the same mistake a thousand times or walked in the acme of obedience all the livelong day. The kernel from which to build-up is this realization. To walk around, one eye on your Heavenly Father, knowing that we are His will and that He values our observations and wants and desires and needs. It's those very things that He's building in you to make you complete.
"For in [Jesus] dwelleth all the fulness of the Godhead bodily. And ye are complete in Him, which is the head of all principality and power:" (Colossians 2:9-10, emphasis mine)