Keep comments to yourself. Unless asked, refrain from giving tips or outright telling another rider what s/he should be doing. Even though a congratulations on work well done is a nice thing to share, a rider is usually concentrating and focusing on the work. Try to save such comments until you notice a rider is doing a free walk on a long rein.
Pass left to left. In general, passing left shoulder by left shoulder is the way to go. Some exceptions are:
- move around rather than inside another rider who is doing a figure as to not interrupt that figure
- if you are moving faster, move to the inside regardless if you're on the left rein. Make eye contact with the other rider you are approaching as s/he may move out of the way.
- always pass with enough room as to not be able to touch even if you tried. (When at home, if you're close enough that I can whip you while you're passing me, I will!)
RIDE PREDICTABLY. Think of the arena like a highway. KEEP MOVING! How you ride should be like clear signals to everyone around you (without talking). Stay far to the sides if you are going slower than everyone else. Ride recognizable figures. Look around you before halting, doing a downward transition, or reining back. Avoid hoarding the circle. (I see riders stick in the same circle over and over and that limits the space others can use. It's no different than having a horse there on a longe line.)
Give room to stallions, young horses, and less experienced riders. If you are at a dressage show and notice a ribbon tied to the top of a horse's tail, that means that horse kicks so give space. A braided horse with a loose forelock is also a sign that horse is a stallion (just in case his bits are out of view!).
Be flexible. The arena is there for everyone's use. Keep your head up and learn how to modify your work without stopping. Doing constant transitions to get out of someone's way is a clear sign that you're not looking around and planning the best route.