If you have a magical item with an effect you wish to save you can substitute it during the crafting process of the new item. It provides 1/2 of the old gold piece value used to make the item as well as providing the spell needed for the purpose of item creation.
In this way you can take a +1 Returning Throwing Dagger apart, and apply it to a +1 Warhammer to make a +2 Returning Warhammer for a discount of 1000gp and providing the spell for Returning.
This is all that is required for making arms and armor; you can apply this same process to potions, scrolls and items but it requires two things;
1) Taking a feat called Arcane Conservation, which also bumps it up to 3/4 of the gp value saved, and
2) An Arcane Alembic for mixing and distilling potions, which costs 5,000 gp