"Just as man is destined to die once, and after that to face judgment, so Christ was sacrificed once to take away the sins of many people; and he will appear a second time, not to bear sin, but to bring salvation to those who are waiting for him." - Hebrews 9:27-28
"You only go around once", "This is a 'one time' offer", "You're only young once", and so the phrases go that tell us about onetime events. So it is with death, in this life, you only die once. There are no retakes or second chances after that moment. Since no one knows when that moment will come, it's best to be prepared.
Jesus died once to take away the sins of many, not all. The fact is, not everyone is ready for death. The most important way to "put your house in order" is to clear your sin debt. Jesus came the first time to do just that; pay for your sin. Only those who have been set free from their sins (verse 15) are truly prepared.
Yes, you only live once in this life, but there is a life to come. Are you ready? This is not a matter to put off until another day. Paul put it well in 2 Corinthians 6:2 "I tell you, now is the time of God's favor, now is the day of salvation." Don't wait my friend, you only die once, so be prepared.