Most of my travel is long-distance. The shortest flights I take are from the Northeast to California, but many have been 24+ hours of travel. Wearing a suit is simply unreasonable. I've heard of many going through security in a suit and changing in the bathroom, for the reasons state on the first page of this thread.
Personally, I normally wear nice slacks and often layers such as a sweater over a dress shirt. I may also do a t-shirt and a blazer. If I get hot, I can remove a layer, if the plane is cold, I can keep the jacket and/or sweater on.
I never wear jeans, not only because the airline personnel are more likely to treat you well if you're dressed nicely, but because jeans are very *thick* and are actually less comfortable on a plane where there is limited mobility and leg room.