I have a personal principle on this issue. If I genuinely have had to prepare at the last minute, then I ask God for help and know that He understands. But then there is a second part to it too—if I have procrastinated and end up preparing at the last minute, then I confess that, ask for forgiveness and still ask God for help.
The first part of the principle has been forged in the relatively gentle furnace of family life and missions organization participation! Sometimes life happens and there is no way to prepare as you would like. God understands this. Last minute preparation is not ideal, but it is possible, and it is still better to prepare as much as you can rather than not prepare at all.
The second part of the principle is there because I am human. I admire people with perfect track records in the area of self-discipline (but I also doubt them!). Rather than make up excuses and try to convince myself that I genuinely could not prepare fully due to life circumstances, I would rather be honest and admit when I have allowed other things, often very good things, to distract me from what was needed as a ministry deadline loomed. I may have lacked self-discipline, I may even have succumbed to some tempting distraction, but I don’t want to succumb to another temptation and seek to justify my procrastination. Hence, I sometimes have to repent and ask for forgiveness and then prepare at the last minute.
May we all be Holy Spirit disciplined in our preparation for ministry and maximize every opportunity to preach the Word. But may we also accept the reality of the grace we preach to others when we sometimes fail to prepare as we should. Not an abuser of grace, nor a rejecter of grace!