1) Practice regularly. If there is one rule for sight-reading, this is it - the more you do it, the better you will become. However, it's not good to practice rarely but in huge time chunks, like 5 or more hours. Instead, it's much better to spend 15-30 minutes a day with sight-reading but do it every day.
2) The level of difficulty. The pieces should be much easier than what you can play currently. For example, if you can practice and master a 3-voice piece in a month, then your sight-reading pieces should not have more than 2 voices at present. In fact, it's better to start so simple, like with 1 voice that you should feel enjoyment and think that it's too easy.
3) Practice tempo. Normally it is very good if you can practice very slowly. But if the texture is very easy and you can do it at a concert speed without mistakes - then it's even better. However, always be aware of how many mistakes you are making and try to eliminate them.
4) Number of voices. It is best to start your sight-reading practice with something simple and easy. This means playing one melodic line at a time. From your polyphonic organ piece choose the soprano alone, then the alto, the tenor and the bass in the same manner. When this practice becomes easy, add a second and a third voice etc. Do this repeatedly for some time and eventually you will be able to sight-read all 4 parts with fluency and ease.
Apply these tips in your sight-reading routine regularly and you will develop adequate sight-reading skills at the organ. 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 Organ Practice Guide.