The right priorities (the core idea, game mechanics and visuals/audio, which supports the former 2) and acceptable preparation (such as BASIC code, where to find sound effects, music etc. for free) is the key in my opinion. Although this is my 1st GameJam, these things helped me a lot, especially because I had a job during weekend (Fr-Sun), so I could work on the game only 16 h (instead of 25-30 h) and was EXHAUSTED. Also the idea was SO persistent, moreover, because I do not like horror games and game like this VERY unlikely be my main project. This game was possible, BECAUSE it was short GameJam.