I also wear yoga pants for my flights overseas. I wear a sport bra and a lightweight sweater/t-shirt combo.
I usually get cold as some point on the plane, so long pants work. If I get hot, I simply pull the pant legs up a bit to expose my calves, and thus cool down.
Absolutely no underwire bras for me on a 10+ hour flight...and no panties at all. I get very uncomfortable with anything binding on my body.
Yoga pants are a heavier fabric (hey, they work great in yoga classes for movement, and for modesty) and work perfectly- even look a bit dressier to me than jeans- when paired with a nice top/sweater and cute shoes.
I don't wear sandals on my flights...again, too cold at times. I wear Puma shoes or something "european" looking and socks. As soon as I'm on the plane, the shoes come off for comfort.
Our bodies swell at 36,000 ft, and I wish to be as comfortable as possible, and thus a bit more refreshed when landing!