....what do you say or do to get their attention nd make them smile just before you press the shutter....
And do you shoot in burst mode?
+1 depends on who is in the group.
With kids I either get the parents out of view of the kids or they will keep looking at them. Or I get the parent to stand behind me and get the kids attention so they are looking at me. If the parents are in the shot with the kids I often asked the kid questions while they pose and get them animated. If they are too young for that I make stupid noises and play peek-a-boo behind the camera. You really have to have no problem with embarrassing yourself to get good child photography most of the time. Parents make better child photographers a lot of the time I think, but I'm sure with practise anyone can get good at it. I don't do studio portraits so a lot of the time I just play with the kids or get them started on an activity and shoot away.
With adults I use lots of tricks, but I'm not always looking for a straight down the barrel type of shot. I don't like fake smiles so I actually try to get people to smile and laugh naturally. There's lots of tricks, a good one is to tell everyone to start fake laughing, demonstrate if you have to so they don't feel so embarrassed. Don't bother taking a shot of the fake laughter, after a few seconds they will usually start laughing for real as it's so ridiculous and even if they don't all do it you will get a mixture of real laughter and smiling.
A good one for groups such as wedding parties is put the bridesmaids and groomsmen into couples that are not real couples. Keep the bride and groom in the centre. Ask everyone to turn and kiss their partner, bride and groom will kiss but the others will turn to each other and start laughing. You can adapt this to other groups of course.
It's really about just having fun with people and getting them to relax and they will start to smile naturally, giving them something to do always helps. I don't shoot in burst mode.
This is a good thread BTW, it would be good to hear techniques from others.