Radiant --

1. I try to teach the same breathing techniques singers use; use your diaphragm, make sure the belly goes in and out, not the chest.

2. You can't direct for it, because you never know where the laughs are going to come. Nothing more embarrising than to stand there waiting for a laugh that never comes. The best I've ever come up with is to tell them to hold for the laugh, then start talking again when laugh has peaked and is starting to come down... don't wait for it to be over, that kills the pacing. I also warn them about walking on the laugh... audiences will stop laughing. One other thing; make your dress rehearsal a family night (or members night)... but get a few people in to watch and (hopefully) laugh. I try to invite people I know who have good, hearty laughs. This is one of those things that you learn by doing... and by making mistakes.

Mike is, of course, exactly right.

The thing to impress upon them is that the throat does not create volume.  Rookies always want to yell, tightening up their neck muscles.  This is a good way to end up whispering and drinking tea with honey all weekend. 

Have them lie down on the floor.  One hand on the stomach.  Then say, "Ha-ha-ha-ha."  They should be able to feel the diaphragm at work.

Then split them in half, one at each end of the auditorium.  Have the groups call to each other, as if they were Sweet Polly Purebread:  "Helloooooo-ooooooooo!"  RELAXED THROATS!  Call, don't shout.  Have them picture seeing their friends on the other side of the football field.  Call to them.  From the stomach.  Full breath.  Use as low a pitch as is comfortable, to keep from using the throat.

Do all this AFTER general vocal warmups; ma-me-mi-mo-mu, mmmm-aaaah, whatever you like to do to get the muscles loose and working.

Stay as far back during rehearsals as you can.  Consciously or unconsciously, they will play to you.

Good luck!

For projection:  have two players stand opposite each other in the middle of room.  Make them have a conversation (in character or as themselves).  Have them take a step a part every 30 seconds are so, until they are at opposite ends of the room.  They will naturally project to hear each other.

For laughs:  I mark in my script the areas I feel most definitely should get a laugh, and by run throughs, I force myself to laugh there.  I also get the cast to believe the audience is another character and then react to them just as they would react to another player.  For some reason, telling them this really works.