Or how to travel without taking the kitchen sink
Lightweight means "not checking a bag". Okay it's not all about air travel, but there are a number of pains in the rear resulting from checking a bag, and that's what first got me into travelling light.
Lost luggage - my case seemed to get lost whenever I travelled anywhere French-speaking. It's probably a Francophone plot against me, or just a natural part of travelling through Charles De Gaulle that everyone else accepts. Yes my bag would turn up a few days later, and be couriered to me, but that's never convenient.
Waiting, waiting - you queue to put it in, and you wait to get it back again. This adds maybe an hour to the hanging around at airports.
Feeling weighed down - it's just an awful feeling dragging a big case around. As soon as you go across cobbles, or want to hoof it to the hostel with a look around the town on the way, it becomes a nuisance to have heavy luggage. Even worse is checking out in the morning and having to haul a case with you all day until you get to the airport in the evening.
So I began travelling "carry-on". I got rid of the sharp things, I slimmed my toiletries kit down, and set about ditching spare garments and items which always came back unused. This took a few trips, each time feeling annoyance at bringing back a garment unworn, before I fully nailed it.
I found I liked the feeling of a small pack on my shoulders as my entire baggage. Seeing people struggle through airports with dual suitcases left me feeling light, free and maybe a little empowered. Watching long-term travellers heft their 80 litre packs and their holdalls (containing all the overflow which wouldn't fit into 80 litres) made me sure I'd made the right choice.
There have been a few occasions when not carrying spares has caught me out. Not normally because of having too few changes of clothes. It's usually the unpredictable - a charger lead failing, or the sole tearing off a boot. In each case I've just gone shopping, and replaced the failed item. I'd have done this anyway, even if travelling with more stuff, because I would never have carried duplicates of absolutely everything.
Backpack - usually about 35 litres.
Eagle Creek travel cube: zip-off walking trousers, mesh top for hiking, shirt, t-shirt (usually used as a nightshirt), underpants x 2, socks x 4, USB lead and charger (maybe two types of charger depending where I'm going). Money & cards stashed in here.
Toiletries pouch: King of Shaves azor4 razor, spare blade(s) (1 per 2 weeks), 5ml shave oil, toothbrush (small kids one), 25ml toothpaste, deodorant gel stick, 50ml mouthwash, 50ml Calypso sunscreen, plastic bag for liquids, hydrocortisone cream 5ml, travel laundry soap, antihistamine tablets, allergy eyedrops., earplugs, spare bootlace.
Travel towel in pouch with swimming trunks.
Passport, driving licence, book, maps, printouts of tickets, baseball cap, sunglasses in hard case, drink bottle, snacks, earphones.
Worn: jeans, shirt, underpants, socks, wallet, waterproof jacket, Android phone, boots.
That's my basic packlist. I'll augment it for specific trips. Often the jacket has gloves and warm hat stashed in the pockets. Sometimes there's a fleece too.
I'm fortunate enough not to wear glasses, so no contact lenses or solutions, and no tampons. Shampoo and shower gel I don't carry - there is usually something provided in the shower, in most places, and if not I can do without for a couple of days or buy something.
Footwear is a space-killer in your pack. I wear a pair of lightweight Keen boots for everything. Occasionally I need to stow a pair of smart black shoes to wear with neat jeans and shirt. Many people advocate taking flipflops for use in hostel showers to avoid picking up infections, but I find drying my feet properly works fine for me.
The other space-killer is electronics. And for each gadget, a charger and lead. I've thinned this out to just carrying a smartphone - a Samsung Galaxy S5 - along with USB lead and charger. For me this works fine as my combined phone, internet device, music player, camera, and torch. It's also my backup documents store. (Second backup is copies of documents in cloud storage.)
To get by with few changes of clothes, I plan on doing a sink wash and rinse of socks and undies every few nights. Done promptly when getting to my accommodation, if hung out somewhere warm they're dry by morning. Every now and then I'll do a proper wash at a hostel laundry; this only takes an hour or so and it's seldom an issue to have a quiet hour reading, or talking to people in the laundry - a very social place.
I find this level of stuff meets my needs indefinitely; it doesn't require more stuff for longer travel, some things just wear out and need replacing. What is a challenge is visiting different climates. For cold places, I need to layer up, though my basic kit makes a good start - for example I'll add thermal long-johns to my walking trousers, and a fleece on top, for wintry places. Tropical is easier - my walking trousers zip off to make shorts, so I just need to pack an extra t-shirt. For wet weather, I need to ensure I'm wearing the walking trousers, along with the waterproof jacket on top. The trousers dry very quickly, often before I've taken them off.
Of course it's up to you how you travel, but this approach works very successfully for me. Happy travels!