Aside from staying within my calorie range for the day, I have this little rule I stick too: If I've already eaten something that day, I don't eat it again that day.
For instance: if I had yogurt for my morning snack, I don't have one in the afternoon. If I had a piece of dark chocolate after lunch, I don't have one after dinner. If I had a huge bowl of strawberries with lunch, I don't have another huge bowl after dinner... you get the idea.
I'm not sure when or how I came up with this, but it has helped me avoid binging (on any one thing at least) or mindlessly grabbing "just one more" and keeps me mindful of variety in my diet. Well, as much variety as a creature of habit like me can tolerate anyway.