I have heard great things about Lululemon yoga pants. Even a friend who would normally never spend so much on a pair of exercise pants said they were indeed worth it for the shape it gives her bottom half.

They are synthetic, of course, and therefore melt in flammable situations if you're concerned about such things. I usually just fly long haul in a pair of comfy chino's which are actually a lot more comfortable than jeans (mine, at least).