Put a folding travel toothbrush and travel-sized toothpaste in the outside pocket of your carryon. When the plane starts to descend, go into the bathroom and brush your teeth. You’ll feel much better instantly. Keep a couple of energy bars in the outside pockets too.
While you’re in that outside packet, throw in a ziplock bag of FAA-approved-size mini-toiletries. Just leave one in every carryon and suitcase. Having it in the outside pocket means you can pull it out to get past checkpoint lanes fast without unpacking the whole suitcase. (Yes, if you’ve got Pre, you can bypass some of these requirements some of the time, but sooner or later, somebody’s gonna make you unpack your toiletries.)
If you frequently drive rental cars and use your phone as GPS, get a vent mount (larger version for phablets/pluses). It’s small, works great, and I just leave that in my carryon along with a phone charger cable and cigarette lighter adapter. It’s awesomely stable.
Roll clothes rather than fold ’em. You’ll end up with less wrinkles, and the clothes pack much tighter. That video shows how to make sure they don’t unroll, too.
Pack with Eagle Creek compression bags. Put each day’s clothing into its own bag, and use the compression zipper to compact it. In the morning, just grab one of the bags. You don’t have to worry about putting together outfits in some dark hotel room.
Cashmere sweaters and bamboo undergarments (socks, undies) pack down really small. Cashmere isn’t as expensive as it used to be.
Bring cash for the hotel maid. Leave $3-$5 on your hotel pillow each time your room is cleaned. Maids are tipped employees, meaning hotels are allowed to pay them less than minimum wage. In addition, make the maid’s life easier by using only a single trash can in the room, and bundle up all your dirty towels in a high location that doesn’t require bending over to pick them up.
Armed with those tips, I can usually get a week’s worth of clothes into a carry-on, it’s so much more nicely organized, and I enjoy my travel more. (For the record, I still use Dufl for my multi-week trips, and I’ve updated my Dufl review with the cool things they’ve added lately.) Update 2020/07/24: Dufl declared bankruptcy and closed up shop due to the COVID-19 pandemic.