1. Take an autograph book - If the lines are not to busy the characters will sign the book at the picture session. Get there early and you have a better chance.
2. Take different posses and smile, this may be the only time you get to take a picture with that character. They do like to get goofy if you do.
3. Don't forget to take pictures of the kids with the friends they have made. They have fonder memories of these people more so than the character in some cases.