I definitely go for comfort on a long flight. There are some track-suits that can be rather chic, however not only sports gear is comfortable. One thing I would go for is breathable materials such as cotton or linen.
Coming back from England I think I'll go for khaki pants and a T-shirt or smarter but still comfy pants with a little top and a casual jacket. As per shoes, yes chic flip flops in summer and comfortable flat shoes/boots in winter.
If you want to be more dressed up when you get to the airport and make an impression, personally I would put a smarter outfit and shoes in a separate bag as hand luggage and change in the labatory just before landing.
I have on occasion travelled on a business suit and heels but only in European flights of approximately 2 hours and because it was work related. On longer flights I find that my feet swell, even with tight trainers (sneakers).