I would recommend talking with your daughter before a party or holiday event, and come to some mutual agreement about how much of a particular type of food she may have. A small piece of cake with no seconds, for example, or three or four potato chips rather than three handfuls. Also make sure she knows which foods she can have as much of as she wants, so that she won't feel that she is hungry, or being deprived.
Praise for sticking with the agreement is good for the first few times, but I would avoid making any type of reward for her eating behavior.