You must hone your test-taking and time-management skills. The GAMSAT, perhaps more than any other admissions exam, is as much a test of your science and communication aptitudes as it is a measure of your ability to play the "test-taking-game". Knowing the rules of this game - how to select choices under conditions of uncertainty, how to estimate numerical solutions quickly without engaging in laborious calculations, how to optimise your score by being efficient but not necessarily comprehensive in selecting which questions to answer, etc. - can make the difference between an average and an excellent score.
MedPrep trains you to play the GAMSAT game.