Here's my go-to solution (this works especially well with little boys- they're very competitive): Say, "I'm gonna race you to the car! On your mark, get set- GO!" If they take off before you say "go", then be sure to make a big deal out of being left behind. Say, "HEY!!! I didn't say GO yet!!! GET BACK HERE!!! NOT FAAAAIIIIRR!!!!" You will have them laughing their head off, and they will be in the car where they need to be in record time.
To be honest, anytime you need any of your kids to do something quickly, just say, "On your mark, get set, GO!" It's weird, it's like this switch hardwired into their brain that when they hear that they suddenly take off.
"Pick up your toys! On your mark, get set, GO!"
"Get your coat on! On your mark, get set, GO!"
Just make sure they can remember what exactly you want them to do or they could end up running halfway down the street before you can get them to stop... ;-)