It’s important to know your test really well. There are parts on every test where marks are fairly easy to get. For example, in the Prix St. Georges test there is a movement at K where you are asked to make a walk-canter transition. That single transition is worth the same mark as five three-tempis across the diagonal. If you do lose marks in one area, you have the chance to make them up with a strong performance elsewhere in the test.
Know the coefficients in each test. At training level, movements such as the free walk and the stretch circle carry a coefficient of two. In Prix St. Georges there are coefficients on the trot half-passes, collected and extended walks, and canter pirouettes. While mistakes in those sections will be costly, they can also be a valuable source of additional marks when performed well.
No matter whether you are competing at walk/trot or grand prix, every test features an entry up the centre line with a halt and salute at the beginning and end of the test. Clearly the entry and halt is an exercise well worth spending time on; doing it well can really improve your overall mark and help mitigate the damage if you have an error or spook elsewhere in the test.