My approach to directing a choir of people who are developmentally disabled is very traditional.  I beat out the timing in the familiar directing patterns. (Others have told me that they find my method to be too complicated and they use a signing method.)  I’m left handed so I’m a bit backward, I beat out the timing with my left hand and give directions with my right hand.  Additionally, I’ve worked out several hand signals that speak clearly to the choir. 
For example, a closed fist means stop, be quiet or “shut your mouth.”  I tell the choir members that if they shut their mouths, it is impossible to sing.  I point to the choir member who is to sing a solo.  I may tell one person or the entire group to be quiet with this hand signal. 
I use my fingers and trumb to make a three-sided rectangle to tell the choir members to sing more softly.  I use this mostly for an individual who is singing too loudly.
When a soloist is to sing I put my fist to my mouth (as though I am holding a mike) to let her know to put the mike to her mouth.  I point to the individual to indicate that s/he is to sing.  To que the person that it is time to take their mikes’ from their mouth, I pull my “imaginary” mike from my mouth and put it to my side. I practice a great deal with the choir with the mikes.  I train them to hold the mikes to their mouth (like a lollypop or ice cream cone) so that the sound person can adjust the volume. I tell them that if they can’t hold the mike to their mouths, I understand completely and my feelings are not hurt. BUT they will not be able to sing a solo because they must be heard to be able to sing a solo.
Perhaps the most important thing that I do is to try and pick music that has longer musical breaks between the vocal phrases so that I can mouth the words to the choir before they sing them.  In this way, I can refresh their memories before they sing the words.  I also use simple signing for more complicated or longer musical phrases.
If a soloist or the choir has a tendency to sing words too quickly or too slowly, I will use my forefingers to punch my cheeks as I move my mouth to the correct timing of the words and music. 
In addition to mouthing the words before they sing them, I also mouth the words as they sing them.  I use exaggerated mouth motions to help them see the words more clearly.  Because the quality of my voices are very different from most of the choir members, I usually do not sing with the choir during a performance.  However, I sing at the rehearsals when we are learning a song, and I slowly wean them from my voice, as they become familiar with the words, by singing more softly, then stopping and only mouthing the words.  It is vital to wean the choir from my voice so that they learn to depend on each other, rather than my voice.
I emphasize the importance of looking at me at all times.  (This is the hardest thing most of our choir members have to do.) I also practice going on stage and off stage.  I tell them they are to walk on and off like a banana peel, not a bunch of grapes.  I feel this prepares the choir and the audience to take the choir seriously.  I don’t allow them to wave to the audience or make other hand gestures to the audience.  I want them to have the feel of an adult, professional choir that is competent in what they do. 
I have what I call my strength triangle.  These are the strongest, best quality voices.  I put them in the center of the choir.  This works several ways.  It disperses the voices that are of bad quality and it allows every person to hear the better quality voices as they sing.  They triangle voices encourage each other to sing with more volume and they keep each other on key.  (I don’t EVER talk about this with the choir.  However, they all know when they are moved into the Triangle).