The biggest mistake people do is to take the GMATPrep mock in the beginning and in the end - very close to the test day. Sometimes it turns out detrimental and they are two days away from test that they cannot even reschedule. Only GMATPrep will be indicative of what your test scores will be. Other tests are just for practice and improve testing experience. What's the point of taking the test at the last moment - it either increases your confidence level or bring it down to zero. You cannot change anything much after that. Instead taking the GMATPrep tests
intermittently will help you improve your content knowledge with whatever time available.
Take a GMATPrep now.
Don't take any tests three days before the D day.
Keep your preparation to minimal in the last three days.
Stay away from GMAT before the test day, do something you enjoy - movies, games, dinner etc
Review a lot - use your error log
If you are facing timing problem - pick a strategy and put it in use.
Get to know your strengths and weaknesses.
Get to know about your instincts.
Don't learn anything new other than honing what you have already learnt.
KUDOS - if my post has helped you.