You might try sanding the inner rod, it will make it so less surface area touches the outer tube making it easier to move. You want a scuffed surface not a smooth one.

You can also do like the Germans do with Bowlen (SP?) cables. You use the outer tube like you would normally, the you put the inter-rod in, but don't hook it to anything. You use a piece of wire inserted into the inner-rod from the servo to the surface, connect them any way you would normally. What this does is it gives you a thin steel rod that is supported the complete run the inner-rod keeps it straight and acts as a sleeve bearing. You get the added benefit of the steel rod being very temperature stable so your trims don't change with the temperature, not much of a problem with a trainer, but in sailplanes trim is everything.