For my trip to Spain via DC-->London-->Barcelona and for most of the trips I go on, I usually go with jeans. While traveling in sweatpants is more comfortable and definitely the surefire way to let everyone know that you don't give a shit what you look like, jeans are still comfortable and imply that you give at least half of a shit.
I also recommend wearing a short sleeve shirt with a sweatshirt or other type of jacket. That way, if you're cold on the plane or in the airport, you can keep it on. Alternatively, if you're sprinting through London Heathrow at 7:30am trying to catch your connecting flight to Barcelona, you can take it off. Obviously make sure you wear underwear, socks and comfortable shoes. But you already knew that. Freaks.
It's also important to have the weather of your destination in mind. While it's snowing and in the single digits in Baltimore, it's sunny and 60 in Barcelona. You need to dress with both of those forecasts in mind so that you're comfortable at all times.
Note: This mainly applies to men's style because I am, spoiler alert, a man. For my readers of the female persuasion, check out Hashtag Jetlag's packing list posts for some options.
4. STACK YOUR BACKPACK
While I already admitted that I'm no packing genie like my mother, I do know a thing or two about stuffing a backpack with everything I need. First thing you need to know is that when you're flying, you don't only get your normal rolling carry-on bag. You also get what airlines affectionately refer to as the "personal item."
They say this has to fit under your seat, but I say F that. As long as it fits comfortably on the ground under you while still giving you some leg room, you're not going to get hassled by any flight attendants. So, I opt for a backpack that I strategically place everything I need in.