I'm in a fun situation when it comes to prepping for my game this Saturday morning. See, for one I haven't had a real day off in about 3 weeks; for two a good chunk of my days have been 12-14 hour days of work, and for three the times I haven't had work I've had other responsibilities. This trend is going to continue right up to driving a housemate to the airport before game this Saturday. I'm not particularly worried though, because I have the "Lazy Man's Prep" to help me get ready for game. How does it work? I'm glad you asked.
Before getting into the lazy's man prep I want to warn you not to do it too often. Your players will notice (trust me, it is obvious ;) ) and that can be bad. That said, it is a good way to handle things once in a while if you truly don't have time to give the next session the prep time it rightfully deserves while guaranteeing that your players will face the sort of challenges that they want.
Step 1: Tell Your Players
The first step is to tell your players that you are doing a Lazy Man's Prep. To help with this each of your players needs to contact you (I prefer email) and let you know what they were up to, what they want to be up to, and what they want to see happen or face. This is why they'll notice by the way, because you are flat out asking them to give you what you need for the session.
Step 2: Collate Data
Next you take all that data and you collate it. Read it over, look for any similar trends in what your players are doing and see what can be done and what can't. This can take time, but ultimately it takes a whole lot less than having to plan out an entire session. Trust me on this one.
Step 3: Bookmark some NPC pages
If you're running a combat game, bookmark some appropriately leveled monsters and enemies in your books. You don't need to set down harsh numbers, you're going to be wining it a lot, but having the stat blocks handy will be helpful.
Step 4: Run The Game
Then you just need to run the game. Keep in mind what the players said they wanted to face and try to bring that up in an orderly fashion. Run combats quick and dirty using the stat blocks you bookmarked. Modify the blocks as you need for variance, and try to keep the fights going a bit faster. After all, you should have a lot of obstacles to throw from your group. Rinse and repeat as you need to get through the session.
Why It Works
This works well for one reason and one reason only. Your PCs set the mission so they know what is coming up, but it also means that it is all stuff that they want to face. No one says "I want to face a dragon!" when they don't really want to. And if they do, they literally have only themselves to blame. So have fun with it, remember that quick and dirty is good when you're improvving it, and try to have fun. The game session will be tiring, you'll be running around like a headless chicken all session, but if you can convey that frantic energy into the session it will all work out.