It is all a bit subjective but personally I felt that doing a few mocks beforehand really helped. You have to be careful as some can be expensive and the exam format is not always very relevant but DS found the experience of having to sit a 2 hour exam in unfamiliar surroundings very beneficial when it came to the real thing.
I wouldn't start the GL papers too early either as they can end up being really boring.
It is really tough now that different exam boards are used. SGS use CEM and although the format is meant to be VR, NVR and Maths, DS found there was a lot of English included. So it is best to cover VR, NVR, Maths and English if your DC is sitting a few exams
Hope that helps and best of luck.