"Be careful with your camera (don't use a flash)...the usher went crazy on a girl who took a photo(long before the show started)"
Or don't use your camera at all.
"I would have stabbed her long before the ushers ever got there. I wish theaters made people check their phones up front, or immediately threw people out who turned on or looked at their phones. They paid for the ticket already, and there are no refunds. The theater loses nothing expect dickholes with no respect for other patrons, cast and crew."
To the first part of what you said, checking a phone won't work. It would cause too much chaos before and after the show. Getting out of a show can be pretty chaotic as it is. Not to mention people are in a rush and the area itself is busy.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611