- Dress modestly, covering shoulders and most part of the legs (women in Orthodox churches might be requested to put on something to cover the head);
- Do not speak loudly - all the descriptions are better either read in silence or given outside;
- Try not to walk too much during the service;
- Avoid using flash in your camera near people you see praying;
- Respect notes that forbid entrance to some parts of the church that are designated as ?for prayer only?.
Following these simple guidelines helped me when I visited, I guess, more than 1000 churches up to date:)))

