1) Tension in the body. When your body is all rigid, shoulders raised, fingers and upper thighs tensed, you cannot play in a natural way. You music becomes also tense and rigid. If you feel some tension in your body, it even gets difficult to breath. We can't relax without breathing and we can't breath without relaxing. So naturally deep breathing is the key for relaxing the entire body.
2) Hitting the keyboard with excess force. I often see organists make this mistake which comes from their pianistic background. They imagine that the louder they want the organ to sound, the harder they have to hit those keys which is absolutely incorrect and may even result in some serious damage to the organ key action. The ideal way would be to play with a gentle mezzo piano touch. The technique which helps to achieve that is keeping the fingers in contact with the keys at all times.
3) Playing the pedals too loud. Organists who hit the keys too hard, often make this mistake as well. They bounce and kick the pedals with excess force which also makes it difficult to control the releases and articulation and may damage the pedal action.
4) Doing unnecessary movements with your body. From time to time many musicians make this mistake - they lean forward or backwards too much. I am not suggesting that the body should be stiff. A little of body movements is fine but too much swinging back and forth may throw your playing off balance.
5) Making agogical accents in the piece too often. This mistake is especially obvious if one plays a short composition imagining that it is a grand symphony. If the piece is short, it is illogical to make those ritardandos and accelerandos very often. Besides, certain type of musical style (early music) does not allow unnatural alterations of tempo.
Remember these signs of overdramatic playing when you practice or perform your organ pieces today. Be careful to use your body in a natural way without tension and without unnecessary movements. Do not use excess force when hitting the keys or the pedals. Instead keep your fingers and feet in contact with the keys at all times. This technique will allow to minimize your efforts and your performance will become natural.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my FREE Organ Practice Guide.
Or if you really want to learn to play any organ composition at sight fluently and without mistakes while working only 15 minutes a day, check out my systematic master course in Organ Sight-Reading.