We have post-its with file names on a wall in the office. You take one down if you edit the file, and put it back up when you're done. Easy.

Though I wish I was entirely kidding. ~12 years ago or so we did that if one of two parallel development teams had to modify a message of the network protocol to avoid incompatibilities and merge problems.

Mind you, these were SVN merges. I can't even verbalize my feelings about SVN merges but by a mixture of laughing and groaning in pain, like if you stubbed your toe in a painful, but entirely funny way.