As a combat archer usually your only defense against incoming arrows is a good eye and fast reflexes. However there are examples from the history of archery in warfare that you as a SCA combat archer can use.
The Assyrians along with having the advantage of iron weapons over their bronze using opponents, made highly effective use of archery in war. The bow was a primary weapon and of major importance in their battles. Unlike the armies of Medieval Europe they provided protection for their archers. This protection was in the form of teams of shieldmen that were responsible for stopping incoming arrows. Early on these shieldmen were one or two men with round shields (approximately 24 TO 30 inch), swords and spears. Or later one man with a tall matlet of reeds, which curved back at the top to provide further protection against descending arrows. This shield bearer was armed and there was often a spearman as well in the team. These were highly effective fighting units both in offense and defense.
This technique can be put to effective use in some of our SCA battles. The combination of one or two shield bearing heavies and an archer can provide protection for the archer from the missiles of enemy lights, allowing him to stay alive longer and take out more fighters, thus protecting his heavies. Heavies attacking this unit can be held by your heavies while you fall back shooting or flank them.
The combination of a lightly armored heavy, javeliner with a pavise and archer creates an effective unit for both offense or defense. The shield and armor of the heavy and the pavise of the javeliner provide protection from missiles for all. While allowing the archer to concentrate on shooting. By using a lightly armored heavy the unit still remains fast and mobile. If attacked by heavies your heavy fights on the defense while the javeliner and archer support him with their missiles. Or if outnumbered they have the speed to fall back rapidly to a protected position.
But the major advantage of this is keeping you alive and shooting for as long as possible, allowing you to reduce the opposing ranks.
Another system of protection used by the Byzantines and the Arabs was the use of a shield on the bow arm. This approximately eighteen inch diameter round shield was strapped to the bow arm leaving the hand free to handle the bow. It provides protection from arrows that you do not see coming, as well as giving an active defense against those that you do see. In lights only battles with boffer or shanai, if allowed, the shield rim can be padded with pipe insulation and used to block sword blows.
For mixed battles, do not wear a shield that could be easily mistaken for the shield of a heavy. You should limit it to the size of a buckler and it should also place the Light Combat symbol on it.
Leonardo da Vinci had an idea for an archer's shield in one of his notebooks. This round shield was about twenty four inches across and had an opening in the center to shoot through and was attached to the bow. A SCA version of this can be made from three eights inch plywood secured to a fiberglass bow with U-brackets. Do not make this too large or you may find the weight awkward.
As an archer you maintain a good defense by: 1) Being aware of what is happening around you. 2) Keeping your concentration on the enemy. 3) Making use of your mobility to maintain the distance you want between you and your target. 4) Using your speed to get to where you are needed or to fall back if overrun. 5) Not looking down at your bow when nocking an arrow. 6) Never dodging in the same predictable direction to avoid arrows (Do not dodge only to the right or left, vary your movement). 7) Making use of cover, both natural, artificial and fighters. 8) Avoiding coming back up in the same predictable place when shooting from behind cover, such as castle or shield walls. 9) Avoiding being shot at from several directions at the same time.
An Archer must be:
Keen of eye,
sure of hand,
fleet of foot,
canny of mind.