I travel a lot with work so like others on here, I keep a travel bag ready to go. I have a full set of travel sized toiletries and make-up brushes that live in my bag. I double-check the contents before each trip.
I also include two universal adapters (multiple phones & laptops means I may need to be charging quite a few things at the same time), a spare battery pack for my phone and a pair of slippers (not all hotels provide them).
I almost always wear black jersey dresses in varying styles. They don't need much/any ironing and are easy to dress up or down. And black hides a multitude of sins, especially spillages. I will add a statement accent (scarf, jewellery, etc) to change the look each day, and I usually take some leather leggings that I can wear underneath the dresses to 1. make it look a bit less conservative and 2. keep me warm if it gets colder suddenly. Layering helps me when I'm moving between different climates on the same trip. I make sure I have a couple of extra outfits in case my stay is extended.
If flying overnight during my trip (this happens a lot), I take my own eye mask, earplugs and a travel size feather pillow with a black silk pillowcase. The pillow is a nice touch of luxury and I sleep better with it. I usually wear a large cashmere scarf when travelling that can double as a blanket if I'm cold. Bose noise cancelling headphones are also a must, as is a phone charger (& adapter for foreign airport lounges).
I take a facial spray, solid travel size deodorant, coloured lip balm, face powder and dry shampoo with me on the flight. And toothpicks and dental floss plus a disposable toothbrush. Having clean teeth really helps me feel fresher. And I apply handcream every few hours.
If I'm taking my Kindle, I make sure I download a few TV shows on BBC iPlayer before leaving the UK.
Melatonin (10mg) is a must on most of my trips (usually a min of a 5 hours timezone difference) as are anti-histamines.
I keep a stash of different currencies at home (for about 20 different countries) so I just add a 'float' to my purse of whatever the currency is where I'm going.
Good quality luggage helps. I buy the lightest I can. It makes a difference when you spend so much time dragging bags on to planes, trains etc. Airline/Eurostar status also makes a big difference and is definitely worth getting if you travel enough.