Ah yes, it has been an issue for me in some exams, including physics.
The first thing to do is to get an idea of the marks per minute or section. Work that out so you know if you're spending too long on one section, though you can afford to spend some extra time if you know exactly what you're doing and that the answer is going to get a decent amount of marks.
If you ever struggle with a question (spending maybe 30 seconds-a minute thinking with no idea), leave it and come back to it later - you may or may not work it out then.
Also, in each question, identify what information is given in the question, and what the answer should look like so you won't make mistakes that way, and write things fairly spaced out so that you can see between steps clearly.
And lastly, do past papers in timed conditions at home so you're used to it.
If you know that 6 marker questions on a random topic always come up for example, then it may be worth planning ahead for what bullet points you might right so you can rattle them off in the exam.
Good luck!