1. You can use Jacotrans and other courier services without any planning on a daily base from one stage to another. My advice is to call them up every time before 8 am and give them name, a pick-up and delivery place, in addition to filling in and attaching the small envelope to your pack. Just call and say "Hola, estoy [name] en el Albergue [name and place] y tengo una mochila para entregar en el Albergue [name and place]" (Every Albergue provides envelopes, often they have a selection of different ones from a variety of providers and the cost for the service is either printed on or can be asked).
2. A few Albergues refuse to accept receipt of transported backpacks categorically. Mostly, however, (the private ones anyway-) will condone acceptance, when you stay the night with them.
Beware of sending your pack just for your pick-up with the intention of staying somewhere else, though, as, understandably, nobody likes that.
When in doubt or in lacking a delivery address, consult with the courier service, but this will necessitate fair Spanish knowledge, as most courier station managers speak little else. Asking for assistance from the hospitalero may also help.
If you stay in pensions, hostals, hotels, etc, you will have no problems at all getting your pack delivered, often enough, it is already waiting for you in your assigned room.
(Note: My personal experience is limited to Jacotrans and some other private providers, not with Correos, the postal service, which is very good, I heard.)