For sciences, scan through the multiple choices and then highlight mentally the long responses and take mental notes those you can answer and those you can't. As soon as the five minutes is up, annotate some information next to the question (what your answers going to be) and then circle those that you can't answer.
For maths, same thing but instead of extra info, it's formulas.
I quickly flip through the test to get a feel on how 'much' stuff is in there (how much writing i need to do). I then skim-read the questions (short answers) so my brains can start to think and answers start coming in. If i have time, I will answer the multiple choice by making a mark with my fingernails.
For maths, if i finish reading the short answers i stare at those crazy last questions and start to figure out what the hell i need to do.
depends if i need to go toilet. if i do, i do. if not, i use the time to visualise my memorised essays, which helps, as i can write without thinking what to write.