make a list on both sides of A4 lined paper listing everything to be packed, often not just a long list but a list with a contents page, footnotes and an index at the back. Also with sub headings (evening shoes, going out pants etc), cross referencing and including items of clothing for every possible weather type and scenario e.g: coats to wear incase of locust plague, ropes in case of tornado or demon invasion, shoes for walking, shoes for sitting, shoes for cobbles, shoes for bobbles, shoes for wibbles and for wobbles. Jewellery for the day, jewellery for the night, jewellery for 2:36pm on a tuesday, jewellery incase you meet the queen, jewellery in case you don't meet the queen, clothes for wearing in the car, clothes for the airport check in, clothes for the bit after the airport check in, clothes for the plane incase of turbulence, snow, drizzle or all three. Clothes to wear whilst waiting for the baggage carousel and clothes that are never actually worn but remain in the suitcase until getting back home.
how many days are we on holiday for? workout this number (using a pencil and the same piece of A4 that has been used by your partner for her list) and take the same amount of pants, socks and t-shirts. Also, a pair of jeans incase a posh do suddenly presents itself and a pair of shoes (flip flops can be bought for extortionate prices either at the airport or one of those strange gift shops that sell them along with flags, postcards and hooky harry potter memoribillia).