Yes, rules! How binding, but following esteemed precedent, only two.
1. Shamelessness. Supplemented of course by politeness, care to not obstruct worship, and gratitude for permitting photographs. Never be annoying, but do be delicately persistent.
Example: Ring the office bell, ask if you can come in, meet the pastor, janitor, a parishioner, etc. In almost all cases churches appreciate being acknowledged, photographed and discussed in a positive light, which helps beat back at least some of the shame. But without at least a little of it, often you just won't get in.
2. Participation. This enterprise could easily descend into mere documentation. Instead, take time to worship or pray (however briefly), and commune with God in these spaces, which is after all what they're for. To use anthropological lingo, ours is both an "etic" and "emic" endeavor.
Example: If the Sacrament is reserved in a Catholic church, treat the host at least as politely as you would treat another person. Even if you don't believe Christ is present there, acknowledge him anyway out of respect for those who do. Why not cross yourself with the baptismal water when entering as well? Likewise, in an Orthodox Church, consider somehow acknowledging the entrance icon.