Sit to accept petting...

You have the right idea! I think the best thing you can do is PRACTICE as much as possible, keeping all the experiences positive, lucrative and not too overwhelming (at first).  Its a a good idea to enlist the help of children but don't forget the adults - work up to those that look "imposing" and others that are Marley's absolute fav's to make it more challenging! 
I'd also practice having Marley sit/down-stay as people knock on the door and come inside.  For most dogs, this will be much harder than a brief sit-stay for the trainer so it should prepare them nicely for the CGC!
 
GOOD LUCK!!!!![] Don't forget to tell us all about the test (Dodger's CGN test involves a handler showing up drunk[:-])

To add to what jetty said, which is pretty much spot-on, our CGC trainer emphasised from day one how important it is to reinforce your dog for keeping his focus and his eyes on you during the exercises. If your dog is totally focused on you, he can not be distracted by the things the CGC test purposely throws in his way to distract him. Our first few sessions of class were basically spent just reinforcing the dog for looking at us and not looking away until we gave them a release word. Having now taken the test, I can really see how key that was and most of the dogs who failed failed because they lacked a consistent focus on their handlers, not because they lacked a specific obedience skill (though the Akita just flat-out refused to sit!). Most of them in fact had much more solid "downs" than Marlowe ever will, but that didn't really help them when it came to the ingoring-distractions/other people/other dogs portions of the test.
 
Fortunately, Marlowe just naturally wants to drill holes in me with his eyes when we're anywhere that makes him a little nervous, so that definately worked in our favor!