Spend at least a day conceptualising your theme ideas before you actually begin coding. That way you have a more certain direction to head in. Save at least two days for bug fixing at the end.
There is a developer I follow that does a jam (Pirate Software) and for that jam you are required to have a design document. I haven't done that jam but I took the idea of that and did a full design doc workup before any coding started. Even though I ended up changing the game to a new premise, I was able to redo my doc and then follow that to make things cleaner and simpler. I feel without that guiding document I would have tried to add too much on.