You can try bribing with money or candy lol... but better (and cheaper) if you learn to "catch" them. Sneak up on them, or when they think you're finished shooting and playing with your cards, but you then quickly look up and catch them. The other thing is to get them doing something they love or maybe even something they aren't normally allowed to do. but in all honesty, it takes time to learn to see the moments and that typically takes lots and lots of clicks -- besides the expressions you capture when you're bribing or forcing them never seem to come out that great anyway (for me). good luck!

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Wendy Armstrong (aka Scrap'N Girl)
my photography site: WendyArmstrongPhotography dot com