My personal rule has been that whatever gear the character was carrying when he shapechanged into an animal would shapechange with him, while whatever gear he picked up while an animal would not.
EXAMPLE: Flavius is wearing red robes and carrying a backpack and a staff. He shapechanges into a horse. THe robes, backpack, and staff shapechange with him. Flavius is then provided with a bit, bridle, saddle, and saddlebags. A few hours later, Flavius returns to the shape of a man. The bit, bridle, saddle, and saddlebags now hang around him, fall at his feet, hang from his mouth, etc.
There are some edge cases where this rule is abuseable but for the most part if plays well. It also seems to reflect how shapechanging is handled in most literature and film. Dracula turns into a bat and his cool cloak changes with him, and when he turns back into Dracula he isn't a pasty white naked due with his privates hanging out.
For more realism you can rule that equipment does not shapechange with the caster, but this tends to lead to a lot of naked casters, and a lot less use of shapechanging.