Stretchy pants (think Gap Stretch khakis) for comfort and lack of circulation issues, tanktop (again comfy), and long sleeved button up shirt (I really dislike the sensation of someone passing by me on the plane and brushing up against my bare skin. This applys to people in my row who insist on taking over the shared armrests and pressing up against my arm.) Shoes are normally close toed because I tend to get cold on planes.
I basically try to look neat and somewhat dressed up since I'm young and I found that dressing nicely helped get me better service than the times I looked liked a grungy college student.