I'm not even kidding when I say this, but for men make sure you're wearing good underwear. On a plane, bad underwear is brutal. I personally wear Saxx Ultra underwear. Incredible.

Also, make sure you're wearing clothes that can be versatile enough to work in warm or cold, as temperature fluctuations on a plane can happen. Such as a nice light jacket, or sport coat, a breathable long sleeved shirt, and comfortable pants. Oh, and shoes that slip on and off easily for checking in.