One bag for a long weekend means no checked luggage, no wait at the carousel and nothing to slow you down between train and taxi. You need a soft bag of about 35 litres that fits an overhead locker, a bed to lay everything out on and twenty minutes.
lay out everything first
Put every item you think you need on the bed before anything goes in the bag. Group it by type: tops, trousers, underwear, toiletries, chargers. Then take away a third. If you cannot name the moment you will wear or use something, it stays at home.

keep to two base colours
Choose two neutral colours, such as navy and grey, and leave out anything that does not work with both. Every top must go with every pair of trousers. For four days, pack three T-shirts or shirts, one pair of trousers, one belt, and underwear and socks for each day.

wear the heaviest pieces
Travel in your bulkiest jacket, your heaviest shoes, the knit and the second pair of trousers. That alone frees about a third of the bag. Take one extra pair of shoes at most, low and light, and lay them heel to toe along the base with socks pushed inside.

roll soft clothes, fold the rest
Roll T-shirts and underwear tightly from the hem up. They take less space and crease less. Fold shirts and trousers once and lay them flat on top. Keep the heaviest items at the base, against the back panel, so the bag stands upright and sits well on one shoulder.

decant toiletries into small bottles
Pour only what you need into bottles of 100 ml or less. About 30 ml each of shampoo and shower gel lasts four days. Put them, with your toothbrush and razor, in one clear resealable bag and pack it last, on top, so it comes out at security in one move.

Packed this way, the bag closes without pressure and still has room for one thing bought on the way home.