Packing for the Big Adventure!

So here's the challenge: Keeping luggage under 23kilos, when we are travelling for three months, across a variety of weather conditions (from Scandinavia in June to the Greek Islands in August); in a variety of types of travel (from 6 star Cunard - complete with formal dress codes - to staying in pubs). What's the answer - one friend suggested the Helen Mirren approach - just take hand luggage and buy whatever you need from OpShops! Or layers and mix'n'match. Anyone got any other ideas?