There's an old joke: "Jesus is coming, look busy." We could argue about whether or not this joke is funny, or even appropriate, but I feel it can be adapted as a useful phrase. Let us rather say "Jesus is coming, get busy".
All throughout the scriptures, Jesus is rather ambiguous about when He's coming back. In fact, several times, He goes as far as to say you cannot know when it will be. This leaves Christians with a question: How do I prepare for when He comes back? Answer: The same way you prepare for every eventuality.
When we get into a car, we know there is the possibility that we will crash. How do we prepare? We put on seat belts. It's possible that we might never crash, but the one time you don't wear it could just as easily be the one time you crash, or at least get busted by a police officer.
This is just one example of a good preparation for an event that might never actually happen. By preparing for it, you're covered both ways. As Christians, we know that Jesus is coming back. But will it be in our lifetimes? Sadly, many Christians have believed He would, only to die having lived by that assumption. Others have fallen for Doomsday preachers who believed they could calculate the exact day of His return, even to the point of basing their financial decisions on it, but of course in the end it turns out they wasted their money and their time. But how do we prepare?
It's simple: Live your life as if Jesus could come at any moment. That way, you're covered both ways. Go to school, get a nice job, get married, have kids, basically just live your life, because you could live to be 120 and still die peacefully in your sleep, centuries away from the second coming. God is patient enough that a millennium can pass in a day in His eyes. But at the same time, He might come back tomorrow. Which would you prefer to say to Him? "I thought I had at least another decade, so I haven't done much" or "here's how I spent the time you gave me..."? Be warned, my brethren, our God is not a marshmallow. Jesus is coming. Get busy.