I will concede that Jaka's approach has its merits. However, to answer the question as it has been put:

Are you using some sort of threaded connector in your linkage? If so, set up the length and crimp so that it's as close as you can get to equal and minimal tension on both sides. Minimal tension means no sagging, but not tight. Then make your final adjustments by turning a few revolutions on your threaded connectors, then lock them with a jam nut when they're taut and neutral. A dab of thread lock will keep them set.

When you do the rudder, the hinge axis is vertical. Gravity does not deflect it one way or the other. Set the tension on either side so the rudder is at neutral with servo at neutral. Now if you roll the model 90 degrees, gravity will pull the rudder slightly down. Disregard this effect, as it will disappear in flight.

When you do the elevators, roll the model so the elevator hinge axis is vertical and adjust tension same way you did for rudder. Now when the model is upright, the elevators will sag a bit. Again, this effect will disappear in flight (or at worst, you'll need to nudge the trim button a bit).