In one sense Christians should be able to preach the gospel in their sleep, in another sense we should never be able to!
What I mean by this is that as believers we are “ambassadors for Christ” (2 Corinthians 5.20), who are commanded to “always be ready to give a defense” (2 Peter 3.15). So therefore, we should be so familiar with the gospel that we are always burst forth its contents forth on our hearers. 
It should be more awkward to restrain the truth from coming out rather than to speak the truth; for this is the fruit of being controlled by the Holy Spirit, as he always draws attention to the truth incarnate (John 16.14).
On the other hand, as followers of Christ we should never just settle into a level of comfort with either Jesus or his message that would serve to undermine the healthy fear that our Master and his job description for us requires:
2 Corinthians 5:11a 11 Therefore, knowing the fear of the Lord, we persuade men,
We should never “get over” the gospel. The reality of standing between heaven and our hearers proclaiming the exclusive means of forgiveness offered to humanity should soberly lead us to a posture of reverence, dependence and humility.
I remember attending a “seeker event” with a local church plant we were helping out with a few years back. While watching the scores of people coming to the event to partake of the free food, games, music and other amusements, I found myself enraged by the spiritual negligence that was occurring. These people were welcomed with open arms and then paraded on out with their stomachs and amusements filled, while their souls were neglected.
After a short period of indignation, I went up to the leader and expressed my concern. To which the leader, with cautious hesitantly, permitted me to grab the microphone. I marched up to the mic and commanded attention and acted as if I had something to say. But, regrettably, I froze. I was angry (probably for the right reasons), but I wasn’t humble. I wasn’t reverent and dependent; I was ready to show them how it was done…all the while modeling evangelism in the flesh.
This lesson has proved valuable throughout the last few years as I have been reminded of the need to pray prior to, during, and after evangelism. For true conversion it is not of me; it is divinely wrought, it is all of God. I pray that God forbids it that I should smudge the glorious gospel with my self-righteous fingerprints!
We must be always ready. And part of being always ready is always being impressed with the glory of Christ and his message, while submitting to his supremacy not only over those who need to hear but me the evangelist!
We even see the Apostle Paul requesting prayer prior to his evangelistic opportunities:
Colossians 4:3-4 3 praying at the same time for us as well, that God will open up to us a door for the word, so that we may speak forth the mystery of Christ, for which I have also been imprisoned; 4 that I may make it clear in the way I ought to speak.
Continue to be impressed with Jesus Christ, that his gospel might be clearly spoken and his glory clearly seen!