What should I leave in the walls to allow me to fish future wires?
This is what ENT (or any sort of conduit) is for

If you want to leave "room for installing wires in the future" in an electrical installation, you put a conduit in the wall so that the installer who is doing the pulls can actually do the pull without getting wires damaged on edges and whatnot.  For data wiring, electrical nonmetallic tubing (ENT) ("smurf tube") is more than adequate -- orange ENT is conventional for data wiring, but any color will do, and 3/4" ENT will provide plenty of room for a data cable or three.  Leaving a pull string inside the conduit is helpful for the installer as well, but not mandatory -- I doubt you'd want to do a long-haul pull through a wall on a pull string, though!