I will translate and abridge here my blog post "lessons from a Jam" that talks about this:
My project was a fail and will be probably nominated for the wooden spoon. . After going to bed late in order to get something I could compile (someone had said me "upload what you have, even if it is a fail"), I read comments such as "this guy joined a jam so he could upload his crap". But I will ponder the lessons learnt:
1) Plan from beggining. I applied the NaNoWriMo principle according to what "planning is not writing". But I did not follow the plan.
2) Before joining, read carefully the date info. Jam started a saturday at 9:00 in my local time zone. But I had planned to start on friday afternoon.
3) Before joining a jam, ask family and friends to plan their week. Or you will find they counted on you to do some tasks.
4) Use a game engine. Some tasks are difficult to do with game engines, but they will help you to do the easy thing easy, and you'll save hours debugging.
5) Use different files for each class / object, since most IDEs don't have the VBA look and feel.
6) Use only one computer. If jam fill find you going from home to home, give up. Sometimes git push says everything is ok, you pull from second computer, you modify the code and and hour later you realize the changes you did in the first computer were not in git.
7) Focus in the important things. Don't expend time planning easter eggs when you still didn't wrote a single line of the interface. Interface is what matters. I spent hours programming room backgrounds (instead of doing with sprites or simply using a white background), but interface is still broken.
8) Focus in the important things. Don't spent time preparing a font to upload it to community for next jam.
9) Don't upload if it's broken or bad quality. They said you "Upload no matter if it's incomplete", but they said it to be polite. Jam Jury is like that of a literary price: they don't want a ton of crap, they want a gram of quality.
10) Don't join jams! They make you lose social life, they made you look bad to friends and family, they make your work pile out, and, after all, they make you feel and idiot...
... but, after all, I had the time of my life. The morning after the jam I solved in minutes some bugs I was unable to solve at late night. But app is still UGLY, HORRIBLE and HUGE. Someday I will upload a fixed version to itch.io or github. By now, I have some tests to check, a nephew to babysit and some shelves to mount.