Wear a sling bag for quick access to your stuff without having to go into your big bag on the airplane.
You probably don't need the fancy laundry soap packets to wash your clothes in the sink. You can use a little bit of laundry soap bar in a plastic bag, or in a pinch, some hand soap.
Nice sandals (like Bedrock) and zero-drop shoes (like Lems) can pack real flat into your bag and reclaim some space.
Bring a Heroclip, a carabiner which can double as a hook to keep your nice backpack off the bathroom floor.
Little A6 notebooks fit almost anywhere and are a great way to jot down notes during your trip.
Soap is always tricky for me. I’ve recently been using Dr Bronner’s. It’s fairly concentrated and a travel-sized bottle covers my showering needs as well as my laundry. The incredibly minor thing I don’t like is that it’s a liquid soap, which is a small chore I have to handle when going through airport security.
Packing cubes are fantastic. They help bring a sense of order to a 35L bag.
Practice. I choose random weekends to pack my 1 bag and live out of it. It really helps build confidence in what you’re choosing to pack.
I was gifted this soap bar bag a couple of years ago, and it accomplishes the same thing. Had I known about how versatile coffee bags were, I would’ve asked for a few boxes instead.
I just couldn’t get the right amount of soap to clean my clothes. I erred on the side of using more soap, which just meant that I needed to rinse my clothes a bit more. I had the proportions down with the liquid form, so it’s just a matter of adapting to a new tool.