Take the time to walk the stadium jumping course and understand what questions the course designer is asking, and how to answer them. Some eventers will walk a cross-country course three times and plan their ride in great detail, but walk the stadium course quickly.
Plan your speed and how to get straight to the jumps; how you are going to use the corners and which ones you’ll go wide at; how you will ride a diagonal line. Look at the terrain and surroundings. If you’re jumping on grass early in the morning, make sure you use large enough studs – the grass is shorter than on cross-country and can be slick. If the jumps are big, bright and more intimidating, they are going to ride much larger than they are.
In the warm-up, most riders just jump whatever is there at whatever height and don’t have a plan. There needs to be a better relationship to what you’ll jump in the ring. Maybe you don’t need to jump four Xs; maybe just one X and then go to a vertical. Maybe your horse needs to school a taller vertical to be careful. If the first jump on course is an oxer, finish your warm-up with an oxer. If your horse struggles with the back rail of an oxer, jump a ramped oxer to get him to stretch across. If he’s strong into a fence, make it a square oxer so he respects the front rail.