Having good time management helps a lot. If you have milestones in your head to have x questions done in y amount of minutes at 25/50/75% or 33/66% of questions it can help reassure you that you are on track, or if you're slipping behind then it's time to catchup. You also have to be strict with yourself with some of the complicated or long winded questions and sometimes you have to guesstimate here to make sure you have time to finish the rest of the questions which will be shorter/easier in comparison.
These were techniques I practised during my GAMSAT sittings and ones I will use again when I sit my UKCAT at the end of this month.