There are a number of ways to have a holy symbol visible without holding it in your hand.Examples include:
A Holy Symbol Tattoo: Just have it on your forehead or other unarmoured part of your body.
The Cassock of the Clergy. Expensive, but uses the Body slot.
The Create Reliquary Arms and Shields feat. Its a feat, requires crafting, but you can churn out holy symbol weapons and armour.
You can also make your weapon or shield into a Consecrated Weapon or Sanctified Shield, which makes them count as a holy symbol. (Adventurer's Armoury)
I don't see any rules saying that you can't just nail a holy symbol to a shield, but I don't see any rules saying that you can, so it's your DMs call there.
As for amulets, I'm not sure whether you can just... thrust your chest to present the symbol.