1 - Unless absolutely necessary never leave the royalty alone.
Don't leave a conversation unless they ask you to. It may seem private, but something may be said or done that you, as retainer, need to know about. If it is really private they will let you know. At times they may have to go to various meetings that you cannot attend. Wait outside for them. Don't go away and plan to come back when it is over; it may get out early or they may need you to do or get something for them. If it is necessary for you to leave them alone while you go get something for them be as quick as possible and make sure they stay where they are so that you can find them again.
2 - Keep your mouth shut.
This has two meanings. First, don't join in conversations that your royalty may be having with other people unless you are spoken to. Keep in mind that when you volunteer to serve as a retainer you are taking on the public facade of being a servant. The old saying "Children should be seen and not heard" applies equally well to retainers. The second meaning applies to what you may hear. As a retainer you will be in situations in which you will be privy to a lot of information which should not be passed on.
3 - Make sure they aren't doing anything they shouldn't be.
A lot of the people who become royalty are very down to earth people who have a hard time accepting preferential treatment, and try to do everything for themselves. While it is nice that they don't have swelled heads about the situation, it does present a problem in that the job of royalty in the SCA is to be a figurehead. It is their job to embody all the pageantry associated with the Middle Ages so that the magic is preserved for all who go to events. As such it is inappropriate to have them carrying heavy burdens, performing physical labor, or performing menial tasks. So keep your eyes out and keep your royalty from hauling all their gear themselves, helping to assemble thrones, or trying to set their own table.
4 - Always have pen and paper with you.
Every time someone gives something to, or does something for your royalty, write it down. Write down the event or the date, who it was, where they are from, and what they did. Every time they are given an invitation to anything, write it down. Every time they make a commitment to be somewhere, write it down. The royalty do so much, have so many thank you notes to write, and have so many commitments to keep that it really helps to have a written record of everything so that nothing is forgotten. Also, write down any ideas you have or suggestions that you may be given on how to make the reign easier. This can make things easier for you as well as your royalty.
5 - Be aware of the time and their schedule.
Carry a watch or small clock on your person and check it frequently. Also, make sure you know what their schedule for the day is. If it is busy, carry a written copy for reference. Let the royalty know when it is approaching time for them to be somewhere. If they have to be at more than one place at once (it does happen!), let them know all of they places they are supposed to be, and they will decide which ones to go to in which order, and which ones to skip if necessary.
6 - Make sure they get enough food, drink, and rest.
Don't let them skip meals no matter how busy they are. If they have to run, make sure they grab a bite to eat of some portable food such as a granola bar. Make sure they drink enough liquids, especially when they are out in the sun. Make sure there is something to drink in court and make sure they drink it. Don't wait for them to ask, offer it to them from time to time. When they actually have free time, make sure they rest. Also make sure you and the other retainers get enough food, drink, and rest. The last thing you want is for someone to pass out in court.
7 - Don't let people annoy them.
If they are talking to someone and that person seems to be annoying them, go up to them, politely interrupt, and remind them of some fictitious place they have to be or appointment they have to keep. If you are already standing next to them, glance at your watch as if you just realized the time, and do the same. Somehow get them away from the source of the annoyance, but be careful about which excuse you use. It would not be good to remind the royalty of a "Laurel meeting", for example, if the source of the annoyance is a Laurel. If you are unsure of whether the person is annoying them, assume they are. If the royalty really wants to be talking to them they can just respond with something such as "Oh, I'm sorry. If forgot to tell you, that meeting has been postponed until three."
8 - Know the signals.
Your royalty will probably establish certain signals for certain circumstances that you will need to know. These signals might mean such things such as "This person is annoying me." or "I need the King/Queen now!" What signals your royalty chooses to define will depend on their own needs, but whatever they are, you need to know them.
9 - Bow or curtsey to tin hats.
If you approach or pass a tin hat you should acknowledge them with a bow or curtsey. The only exception to this is when you are part of a procession. If you are part of a procession, don't bow or curtsey to tin hats that you pass, but do bow or curtsey to any royalty already upon the dais as you reach the head of the procession.
You should also learn to recognize what the various tin hats mean. Six pearls or projections indicate a Baron or Baroness, whether court or territorial, and both are addressed as Your Excellency. Twelve or more pearls or projections indicate a Viscount or Viscountess, also addressed as Your Excellency. Crenellations indicate a Count or Countess, again addressed as Your Excellency. Strawberry Leaves indicate a Duke or Duchess, addressed as Your Grace. If you don't already know them, have someone point out the various crowns and coronets for all the Royalty of your Kingdom. The King and Queen are addressed as Your Majesty. Princes and Princesses, whether they are the Kingdom heirs or the rulers of a principality, are addressed as Your Highness. Heirs to a principality are addressed as Your Excellency. It is also a good idea to learn the crowns and coronets of the royalty from other Kingdoms, especially bordering ones.
10 - Never forget you are here to have fun.
If things start to get to be too much for you, don't just suffer silently. Let your royalty know. Overworking yourself won't make the reign any easier for anyone, and is likely to sour you on the SCA. It is much better for everyone involved if you cut back on your duties and let someone else take up the slack, rather than burn yourself out.