Practice your timing
This tip is vital. You could have everything else planned and prepared to perfection and still end up falling at the final hurdle if you do not consider timing! History may be one of the most demanding subjects out there with regard to timing, having to write 3 long essays, 1 short essay and answer a comprehension all in the space of 2 hours and 50 minutes and so it’s crucial that you know the exact length of time that it takes you to write out the content. You should dedicate no more than 40 minutes to each question.
If you find that you're going over the limit of 40 minutes, cut the essay down. Remember that this is not an English essay, so no need for flowery prose and filler language. History essays are fact-based and you should easily be able to fit an essay into the given time.
On the day of the exam, the first tip I would give would be to bring a watch with you. As previously mentioned, the exam is heavily time-dependent and so you do not want to be wasting precious time craning your neck and squinting your eyes to the giant clock on the wall of the exam hall!