When you’re jamming all your clothes into your suitcase/rucksack, don’t fold them neatly. Roll them up. You’ll find you can cram more into smaller spaces that way. This can be especially useful if you’re flying RyanAir as they effectively offer a few quid discount on a flight if you only use carry-on luggage. Smaller space = smaller bag = possibility of taking it in the cabin and saving some money.