You can answer your question my looking in a mirror, say "CHEESE" and watch
your mouth. It's hard to say an enthusiastic cheese without smiling. ;)

Then again just plain old cutting the cheese works too! At least with kids.

I think the point of 'cheese' was the long eeee sound which kept the
mouth in something vaguely reminiscent of a smile.  I can't recall ever
using it despite shooting a lot more weddings than I care to remember.
I prefer to try to get them grinning or laughing at a lousy joke or
stupid word -it depends on your style and the circumstances - for
younger subjects I use 'elephant'.... usually gets a good grin.  For
adults, 'sex' works pretty well, although perhaps not at a church
fair...

(I hope that last sentence doesn't get quoted out of context..)  (O:

back in my kiddie pix days when it was required to produce those 'cheesy'
smiles, that was the worst one to use, cause you know mommy practiced it
with them and it can only produce the most godawful grimaces where the kid
looks more like he's imitating a hissing cat.

whiskey was usually good for a fun reaction, unless the family was Ned
Flander's christian then they would stop the proceedings while they
explained they don't use such language.

money

smelly socks

pizza

and don't ask me why, and I don't even know how I came up with this one, but
it works surprisingly well..

rudabaga

???  I mean, its a vegitable no one in america that eats out of microwavable
boxes even knows what it is, if they did they'd know it wasn't a 'fun'
veggie, down there with turnips, parsnips and beets.  Ok, pumpkins are fun.

But you know the best way to get kids to smile is to not GET them to smile,
let them play, give them an activity to get into.  I know that doesn't work
in a production, high volume studio, you have a line of mothers,
rambunctious kids flying around loose, fussy babies, whinning babies, crying
babies, shrieking wraiths from....(snap snap, phew, there, that's better,
sorry I was having a flash back to my kiddie pix days.)

Take them to the park or beach, let them play, build a sand castle, you're a
block away with your 200mm with the kids compositionally placed in the lower
power corner, so there's no camera to freak the little one out.

sure fire way to get smiles, get a bushel basket to put the baby in (also
handy to carry the props, like a bushel of apples...plastic ones as they
keep longer, or blocks, or teddy bears) and have the toddler give the older
kid an apple, another, and another, pretty soon the older kid has his arms
full and can't hold them anymore and starts dropping them, I guarantee you
everybody will be laughing.

this reply is echoed to the z-prophoto mailing list at yahoogroups.com

In my experience it gives a more satisfactory result to ask the subject
to "say sixty-six"

Didn't work for me - but what did work fantastically is getting them to
laugh, and then firing off a quick burst of shots.

I even tried one of those CDs of "rude noises" etc that was pretty good too.

Always found it best to setup all the gear and use some stand ins for kids
first (to check levels etc) - getting a few laughs isn't too hard, but if
you don't get the shot initially, then you'd better be one hell of a
stand-up comedian if you're going to keep them laughing :)

These almost always work:
"Chicken fat!"
"Spaghetti sandwiches!"
"Hamburger noodles!"
etc.

Non seqiturs work best, especially if you use a different one each
time.

As for teenagers, you're doomed.