Tie a piece of string to the tripod that you can stretch out to the subject and use as a last second focus check. For instance, have them (or a stand in) hold the end of the string to their cheek while you set critical focus, and then when the darkslide is pulled, and everything is ready to go, stretch the string out again to get them at the perfect distance. This saves them having to try to hold still for a long period of time. Most people who aren't models, or who aren't used to the focus issues LF'rs deal with have a limited concept of what "don't move" means, in our context. When they move, it is almost always front to back as they relax, get bored, get interested, or begin to "get ready" even though they think they are "holding still".