+1

Comfortable but not sloppy. Many years ago I used to consistently see attitudes and different levels of service for people dressed in shorts and T-shirts, but it's common these days, especially on overnight flights. The world is a more casual place in general. In the summer I usually go with shorts or jeans, and a T or polo shirt. A good rule of thumb might be to wear something you'll be comfortable in at your destination (unless you are going right to a meeting...no one expects a suit and tie on an 8 hour flight!). My rule is no gym shorts, sweats, flip flops, or anything that casual.

More importantly, it's about your attitude not your dress. Be polite to the crew and your neighbors. And a good piece of advice a football coach told me nearly 30 years ago (when scoring a TD)..."Act like you've been there before".