The most expensive mistake in a subjective paper is not a weak answer - it is an unanswered question. The first marks in any answer are the easiest to earn, and the last marks the hardest, so an incomplete paper with three strong answers almost always scores below a complete one with five adequate answers.
Work out your minutes per mark before you start writing, and hold to it. If a question is running long, close it with a short conclusion and move on. You can return to it if time allows.
Leave a few lines of space after each answer. If you remember something later, you can add it where the marker will read it, rather than on a loose page at the back that may never be connected to the question.