The best way for me is to trim the plane is for One speed. The more you can fly at any one given airspeed, the less one has to work at fighting the airplane. Some will say that you have to pull to the vertical and trim there, but the airplane doesn't know which way it is going. Flow is everything in a sequence. The trim will change with changes in airspeed. This is known as P Factor, thus one speed that you try to attain should be the happy medium between Max Airspeed Vne and Vso Stalling speed in a given configuration. In this case it is clean stalling speed. The airplane will tell you the air speed it is most comfortable at. About half throttle in level flight is a good starting position. After you have this speed, try and fly the same speed for the entire flight. If you do this, your rudder trim should never change except for when you accelerate or decelerate. P Factor and Torque now take over, and you as the pilot must compensate. I know what you are trying to get the answer to, but you have to learn to fly the plane like it is meant to be flown. By this I mean put yourself in the airplane and visualize that when you do one thing, something else has to change.
If you can get the book Advanced Aerobatics by Mike Goulian and Geza Szourvy, they have a great chapter on Gyroscopic Precession. This will help explain the answer you are looking for.

Hope this Helps,
Mark Leseberg jr.