Ticket collection when abroad?

Planning a trip in high travel season (August) so I wanted to book in as much advance as possible. But! We wont be in China for a month before that. Plus I'd rather not inconvinience any of our friends.... Does anyone know how long a rail office will hold onto our tickets for pick up? Does it need to be done immediately or should we book through a travel agency? If so, does anyone recommend any travel agency in particular? My Chinese is intermediate. Thanks