Tips for syncing audio with video?

Budget restraints have me recording audio separately (Tascam DR-10L) from video. I do the clap or click thing to visualize a db spike in the audio track, but I often turn the camera off and on, or need to stop it for a moment, while keeping the lav mic going. It takes a long time to resync in post, after that.

Do you have suggestions for anticipating this in production, like using a stopwatch or some other way to mark lapsing time to use later?