(1) Get good sleep. Be rested, alert, and energized for your exam.
(2) Use your scratch paper wisely. Before your exam starts, write down any concepts that sometimes bother you (or anything you think might be helpful) to get those things out of your head so you can relax.
(3) Turn all of your memorized equations into sentences. For instance, F = ma can become "when an equal force is applied to two objects, the lighter one accelerates more." Or, "it requires a greater force to accelerate a heavier object from rest to 10 m/s than a lighter object." That skill is paramount to doing well in the science sections.
(4) Get into a POE state of mind.