I've found real sign language (ASL) to be very useful sometimes during directing. More than anything, it can be a helpful reminder for the choir at those places in a song where it's easy to forget what lyrics come next.
For example, one of the choirs I direct sings "King Of Love" by Hillsong. On the first verse, some of the singers have trouble remembering which word comes first -- "marvelous" or "wonderful". When we sing that part, I do the ASL sign for the letter "M" first, to remind them that the first thing they say is "You're marvelous". Then, at the next section, I sign the letter "W" for them to sing "You're wonderful".
Other signs that have come in handy during some of our songs include the signs for "love", "heal", "and", "yes", and others. It's a very good alternative to mouthing words to them to tell them what's coming next.
Even if the members of the choir don't know sign language, during rehearsal you can teach them the couple of signs that you plan to use during a particular song.
If you want to learn a few signs, there are sign language books that are written specifically for church ministry.
Do you ever use ASL while you're directing? What signs do you use the most?