I've done a lot of past Ludum Dares and each time I manage to finish something because of some simple preparation steps.

1) Prepare your tools

You need to make sure your tools are up to date and functional. For example, I use Unity, and certain build options require some setup. I usually build for web as my main target because that is what will get the most plays since it does not have Operating System compatibility issues, doesn't require installation, and is quick to link to. Knowing where you intend to upload your target builds, how to package them, and how to use the Ludum Dare submission save you time and stress.

Whether you build your own assets or use free ones also can save time as you can seek out resources to build your game BEFORE the event. If you don't intend to make your own audio for example you can find some assets and just be sure to mark your submission as not graded on audio.

2) Prototype some games

Once you've done a few game jams you should have some source code to reference and some design ideas. You will be able to reduce time spent trying to figure out how to do something.

Movement controllers are highly reusable and it helps to have some to reference.

Input controllers are also highly reusable, knowing how to select, grab, move, etc can be used in many different types of games. Troubleshooting these controllers can be extremely time consuming and are likely causes for new jammers to be unable to submit a working game.

During this prototyping, if your engine supports it, you can try out free assets. Cinemachine is built into Unity and offers an advanced camera controller with little work needed.

By prototyping some mechanics or systems you get nice reference code to use and solve future issues you would have come across.

3) Budget some time by doing it now

Do your grocery shopping, mark off the weekend on your calendar, clean up your workspace, and get chores done before the start of the jam.

Something worth planning is also finding some useful things to do when you need a break. The best games are not made by people that spend the full 72 hours coding away. People need breaks, but it's also easy to just get lost in what you do on your break, just one more game right?

Also worth mention