Airlines have made flying so frustrating that I can understand why people just want to jump out of bed and travel in their jammies. But I strongly encourage you to think twice about that for three reasons:
1) People will stare (in a good way) -- While wearing this cute Betsey Johnson cotton dress (photo) to and from my Minnesota vacation (yes, it's alright to wear the same thing twice) I got a total of 8 compliments. While most of the other women at the airport sported shorts, jeans, tank tops, leggings, yoga pants, t-shirts, and pajamas, I dressed and looked like a lady. And I got treated like a lady!
2) Be comfortable but be a lady -- Loose fitting, light-fabric (think cotton, linen, silk) dresses pack well (roll them instead of folding them) and wear well. Collapsible ballet flats take up very little space in a carry-on bag. A pair of cotton white capri pants go with anything and can be dressed up (if going to a nice dinner) or played down (walk on the beach) much easier than shorts. A bright, sleeveless silk shirt will always look classier than a jersey knit tank top.
3) Don't let the airlines win -- If we start to let ourselves go (fashion wise) because the airlines have made air travel so difficult (smaller seats, smaller bathrooms, no luxuries at all), then we are now the proverbial fashion lemming who let's them think they dictate our lives because they dictate our travel. Don't let them win your style game!