The gap between “weekend bag” and “suitcase” is where most people get stuck. A 40L pack sits exactly in it, and once you have packed one properly you will find it hard to go back.
Start with the shoes
Shoes decide everything. Wear the bulkiest pair, pack one flat alternative, and put it in the dedicated shoe compartment if your pack has one. That single decision usually saves 6 – 8 litres.
Roll, then compress
Rolling beats folding for anything knitted and loses to folding for anything structured. Roll t-shirts and underwear, fold shirts and trousers, then use a compression cube for the rolled pile only.
The 4-3-2-1 baseline
Four tops, three bottoms, two pairs of shoes, one jacket. It covers seven days if you do a sink wash midway, and it fits a 40L pack with room for a laptop.
Keep the flat pocket flat
The laptop sleeve is not storage. Anything you add there pushes into your back and turns a comfortable pack into a bad one by hour three.
Weigh it before you leave
A packed 40L bag should land between 15 and 22 lb. Above that, the carry-on advantage disappears and you start looking for a trolley.