The first method of routing an inner cable through the tube (besides the liner sleeve already mentioned) that I use is to put a slight kink in the inner cable about 1" from it's end (this allows you to roll/rotate the cable and it's end will flop around a bit). This and if the, usually, rear cable port plate is removable a tool made from a spoke in the shape of a "J" to act as a hook reaching in with.

These usually do the trick, although I have had to hold a flashlight in my mouth while I reach inside with one hand and spin/move the cable with the other hand.

Have I said lately how much I hate internal routing??? Andy.