If you have a choice of questions, opt for the one where you are most confident with the tense needed. Make sure you use these to your advantage. They often contain keywords that you can use in your answer. Answer the question, don't translate it. Know your own limits.
Mnemonics are great for the Higher paper.
Then try and make up your own mnemonic. Write it in the margin at the start of the exam, and try to include these four elements somewhere. You can use the same principle for checking for mistakes. Given below are some sentences containing many of the commonest mistakes made in the writing exam. Identify the common mistake in each.