Well TIP actually. There is only one.
If you're shooting a rain scene, leave the underwear in the trailer.
You can either "go cowboy" to set or you'll be forced to "go cowboy" home. Because nothing sucks worse than the moment you realize your underwear is wet. And now they're moving the camera so you have about twenty minutes to sit in your wet shorts.
It's times like these when I am forced to turn to my fellow cast mates and remark, "Isn't this glamorous?" "Truly the greatest job in the world. It's so glamorous."