Cable guides for CAT 5e in the rafters?

One easy thing to do would be to just get some 1" or 1.5" flexible plastic conduit and fasten it to the rafters with pipe hangers. This way, you only need to go into the attic once (to install the conduit). All subsequent cable runs can just be fished through the conduit. This type of conduit is readily available at the big box home improvement stores.

I don't have enough points to comment yet, but I'd like to add the tip of pulling some fishing lines through the conduits to help pull cables at a later date - in my experience, cables have a tendancy to get caught on the ridges in flexible conduit.

Being able to tie a new cable to some fishing line, then go to the other end of the conduit and pull the cable through is a big time saver. When you pull a new cable, tie some fishing line to that, so it leaves a new line for the next cable, and so on.