Somebody mentioned this already, but it really can't be stressed enough. Not so much a secret, but common sense that people unfortunately tend to forget. CM are human too. They're working long days, it's hot, they don't pick their costumes so they might be in long sleeves or thick clothing on 90+ days, they're on their feet all day, and they have to deal with lots of people that treat them horribly. A little bit of kindness goes a long way. Just some basic courtesy. Say hello back, have a bit of a chat if they're not currently dealing with a situation (ex: chat with someone working an empty gift shop, not with someone trying to load people onto an attraction), just be nice. You get something in return, you might not. But even if you don't, you made someone's day a little better and Disney's all about making people's days better. It's the little things.
Extra tip here: a good starting point is to take note of where they're from. Visited their are recently? Mention it! From the same area? Tell them! I can't even keep count of how many times my dad and I have gotten things here and there from that. Last time we were there (November 2014) we hit HS on Thanksgiving and went to RnR only to find it was temporarily down. We noticed the CM at the front line was from PR just like us so we mentioned it, wished him a happy Thanksgiving, thanked him for working holidays so people like us could have a magical time. We wound up getting about 12 fast passes from the CM. Some paper ones that worked on any of the rides, anytime. It was amazing. So remember, be nice to CM! It's the least you can do after they're working so hard to give you a magical vacation.