Once the CG is set correctly on your practice model, set up your competition model the same way. If your model is landing without the propeller turning, increase the length of the rubber loop on subsequent flights to insure power all the way back to landing. Maximum flight time will be obtained this way. Experiment with different propellers. Keep good records in a logbook and only make one change at a time for observation.
Champions build two or three models at the same time.
The first model can be used for experimentation on wood, building technique, and flight adjustment practice. Sometimes the wings, stabilizers, or motor sticks can be interchanged to find just the right combination for a superior model. Champion modelers experiment and change things a lot. Trim out your best model for maximum duration, mark the wing posts with a pen so that the removable wing can be reinstalled perfectly every time, and fly the best model a few times for practice before putting it away for competition day. Use the other two models to practice flying, to learn how to load the rubber motor, and to learn how to make good flying adjustments.