I only wished to add a bit more variety to the posts around and giving some tips to those who are starting with their first jam.
In less than 9 days (from this post), the theme will be displayed and we will all be able to start jamming.
Have you though about what kind of game you'll work on yet?
Even without a theme, you can still think of the kind of game you'll be working on and that's applicable before the dev jam even started. A theme usually doesn't decide what kind of gameplay you'll be working on. Kinda like it's possible to have a racing game of any kind of story/setting/styles or having a fighting game with any characters, knowing what you'll be working on ahead is crucial at making the experience a good one instead of just a stress-filled bad one.
I already have 2 kinds of games and 1 partial kind of game in mind with notes of each on notepads I have around. Depending on the theme, I'll select the kind of game that fits the best in my opinion once said theme it's revealed.
Have you looked online for sources for open source assets yet?
There's nothing stopping you from looking and downloading open source assets ahead of the start. Last thing you want is to waste hours on looking for generic stuff you could have found days before the jam. The most common assets dev jams devs usually lost the most time on are audio files such as sounds. Open source sounds aren't always good one or lacks in quality and, in many case, might not even fit with your needs so having a bank of those ahead of time can save you a lot of trouble. Obviously, you don't have to download everything available online so you should, at the very list, make yourself a list of bookmarks for sites (or search results on sites) for your particular needs you think you might have during the jam.
There are a few links on this discussion board you can look into. You can also search for your own sources too.
Have you prepared yourself in real-life for the devs jam?
Usually, everyone has a life outside of the jam, so it's never a bad idea to prepare yourself for the jam. Make sure you got food available for those 10 days. Make sure your coworker knows what you'll be getting into. If you got a family at hand, explain to them what you'll be working on. Not only they might help you (alleviate the family-life style), you'll be their hero once you're done with your game. They can be good source at finding issues you might have missed in your project too.
Practice while you can.
If you're not that much occupied yet, you can work on ideas and concepts ahead of time. There's nothing stopping your from creating a prototype of your project right now and test things out prior to the start. Kinda like how an 100m athlete does practice runs all the time and not just during competitions, keep practicing your devs skills so that, once you're in the competition you know what you're doing and avoid wasting time on "what if" and "how should I do it" questions. True, once the jam start you'll have to start from scratch, but the hardest part of game development is actually having a clear idea of how the game is played and how it's coded behind. If you already know how it's played and already wrote your own code for it to work, you just have to write it again during the jam and you'll have more time at adding more cool stuff into your project.
Set a proper time sheet and workflow
Let's be honest, we all hate looking at a clock while working on anything. Time either goes too slow or too fast. This is why you should prepare a time sheet with your main goals set within the 10 days frame and think of how you'll achieve those goal (workflow). How many hours per day will you work on the project? How much rest will you have? How much time you need for each steps? What's the B plans in case you fails to reach your goal, if any possible? 10 days is really little in game development and you might consider actually having 8 days since there's nothing telling you that everything will work fine when you'll build your game for submission. You might loose 1 or 2 days on that alone!
I know... it happened to me twice out of 3 devs jams I worked in. Issues with unknown limits within the core of the Unity engine generated major game breaking issues in my project only on specific kind of builds.
A tip for submitting your game source and build
When submitting your build, make sure you clean it up as much as possible. In the source, you don't have to includes the original prepared source files if they are not used in the engine itself. Clean up any files that are NOT in use. You might think that the engine will clean up the unused files when building the client files, but that's not exactly true as there's a particular setup to follow for it to work and this setup can break your game. Having the build purged of all unused files makes the build faster with lighter files and it's easier to share your project source files. The judges don't need to see your 6 version of a file if you don't use those 6 in the result. If you use Photoshop's PSD files in-engine, turn those into PNG or JPEG or whatever you want that is lighter. Remove unused audio and image files. You might not know it, but both Unity and Unreal don't purge any audio and image files (even unused ones) because those can be loaded dynamically through scripts and their compiler aren't smart enough to detect dynamic references. (Well, there's an option to purge all unused files types by force, but that breaks 90% of the projects so I don't consider it an option.)