I found that I did better on exams when I didn't worry about planning as much and just sort of let myself type. By the time I was taking the exams, I knew the material well enough that arguments, objections, etc. would just come to me naturally as I wrote. That IMO leads to a much better exam than whatever else it is people do. Just write, and keep writing (obviously this is for an in-class exam w no word limit).
I hate science because I refuse to assume that a discipline based in large part on the continual scrapping and renewal of ideas is unconditionally correct in a given area.