What really helped me finish was that I intended to only work for two days on my game and just upload whatever gets done in that time frame.
But when testing the game I liked it much more than I had anticipated and decided to keep working on it. At that point I already had a functioning game but still had 4.5 days left, which was more than enough to add more content.
I think I would do it the same way again if I ever participated in another game jam. If I don't enjoy playing my own game after two days, it would not make much sense to keep going. And if I like it, there is plenty of time for adding more content and polishing the game.