Always put something down - especially in tests incorporating multiple choice answers.
Read the question and you think you know what you have to do.
Read the question before writing the answer.
A few random guessed answers can not do any harm.
Do not spend too long on a question. It will be a remarkable question that will trip you up completely.
Look for a different approach if you cannot see a solution in a `reasonable' time.
When you can look back over your answers. Concentrate on questions you may have found hard.