First off, it's important to plan a letter. There's a structure to be followed and if you stick to the rules set, chances are you'll score a good mark.
Here you find some tips on how to minimise making mistakes.
- Study the structure by heart. Make sure the month has a Capital letter and Dear Mr/Mrs, is followed by a comma and more such rules.
- Think about the structure before you start writing. How many words are allowed? Which items go where? Use a draft version to draw up your "starter" letter and allow yourself to make mistakes now. Only when you start your last version will you pick up your dictionary and look up unknown words.
- Do not make sentences longer than you can handle. It's no use doing so, since you probably will make mistakes which will annoy the reader. Best stick to what you know.