One of my favorite techniques which help increase my focus is playing in a noisy environment. This could be other organ music sounding at the same time when you play, a street noise, people talking or some other sort of sounds. Playing without the organ stops on works well, too.
The trick is to force yourself not to pay attention to the outside noise. Instead, put all your conscious effort into your current measure. The current measure is a life saver in organ recitals or church services.
Whenever you feel like losing focus, you should force yourself to stay in the current measure. Your eyes should be looking straight ahead to the score at the exact spot you are playing. Remember, we focus through our eyes. Therefore it is of utmost importance to truly sink into that measure you are in the current moment.
Don't try to think ahead about approaching difficult episodes. They have not happened yet. Also don't think about the past places in your score. They are already gone. What matters now is this measure only.
To make your focus stronger, you can also put your headphones on with some music, radio or other form of sounds and noise. This way you can create a noisy environment anywhere you want, virtually anywhere you practice.
By the way, you don't have to do this kind of practice at the organ, either. You could practice this way anywhere there is a stool and a table. The stool is needed for you to sit down and the table - to put your music score on.
A practice without an instrument at the table is a superb way to force you to focus your inner mind. Without the real organ sounding while you are pretending to depress the keys, it is virtual impossible to do it without a real focus.
You can go one step further with this and put your headphones with music, talk, radio, or noise recorded. You will hardly find a better way to force yourself to focus.
Apply these tricks in your organ practice, if you want to increase your focus and reduce the number of mistakes while playing. Of course, as with anything worth pursuing this is not easy at first, but if you stick with it long enough, the rewards will be enormous.