1. Take care of business - Fire up the computer then get yourself a drink and something healthy to snack on; attending to biological needs is the first step to solving the problem and removes any distractions. Plus the very act of doing these things gets you out of the 'must prep' funk you've put yourself in. Take about five minutes if you can as this is the lead up to your main objective.
2. Write down what you want to achieve - By this I mean something like 'Run a fun and entertaining game session with my friends (insert names here).' This is a declaration of intent and you'll refer back to this later. You need only write one sentence per minute on this and can only spend three minutes tops on it. This will help your focus if you find your attention drifting.
3. Look at your notes - Unless this is a new session, your dog was super-hungry or (insert crisis here) destroyed your notes, you have source material already at your disposal. This includes any copies of character sheets you have available so get up to speed - spend no more than seven minutes getting a feel for things and don't sweat the small stuff up to and including the Maoist Revolution.
4. What shall we do tonight, Brain? - You know your players/audience. Have you already got something you can adapt like a module or sourcebook encounter into maybe three to five encounters? One of the strengths of 4E D&D is prep of this kind is very easy to do - even lazy DMs can handle it. Keep an eye on the strengths of your party and remember to adjust for balance - you have 20 minutes.
If you don't - feel free to go cross-game/genre and amend some minor details. Does chasing a werewolf wizard arsonist* as she sets fires in a town to conceal her plan to awaken a fiendish vampire cleric sound like good 3.xE fun? Who cares if it's a World of Darkness scenario? Do some converting! You get 25 minutes with this option as it's a bit harder. Help is available!
So you're away from most of your stuff? There are plenty of online resources even if you're strapped for ideas. I've referenced Abulafia before and will also flag Roleplaying Tips, WotC's Map-A-Week Archive (maps kill me) and the blogosphere has many ready-made characters, items, locations and monsters for you to work with - as you have the most work to do, you get 30 minutes.
Some sites provide additional system-specific help including D&D Insider and the d20 SRD. Plenty of other examples exist for your system and Google (or some other search engine) will find them for you once you've read this post. Don't try to find them in the hour prep as you need that time for everything else. Feel free to leave your personal favourites in the comments.
You need three encounters minimum as people have worked out that the typical gaming session (about 3 hours) goes through an encounter an hour. I would aim for four or five ideally, which makes the five room dungeons from Roleplaying Tips a particularly handy resource as you can springboard off those or even - gosh - use them with some personalised tweaks.
5. Tweaks and Session Preparation
Take a look at the result of the preceding steps in context of step 2. Your friends need an individual moment in the sun so check what you've put together can give them a moment to shine. Ten minutes will hopefully sort this out - you have an extra five minutes if you have pre-generated materials handy as you'll need time to familiarise yourself with subtle nuances your original reading missed...
6. Get organised - Get the books and materials (miniatures, maps, notes and dice) together; if you're hosting then get the room set up as you need to; this will hopefully take about 5 - 10 minutes. Take a couple of deep breaths, roll your shoulders back on each breath and smile as you exhale. Remind yourself these are your friends and that you're here to have fun.
There. One hour. You've had time to tweak the material, review it for fit and possibly improve on a classic design. How often does that happen in your usual preparation? Now go kick some ass!
* - Rumours of her name being Samantha Hayte cannot be substantiated.