We always take one shirt, pair of socks, underwear for each day, then one for spare. Then a pair of jeans or combat trousers each for every four days.
Years ago I wrote two check lists which I've saved on the computer. One is for travelling abroad and the other for in the UK where we can buy things more easily. I then print a list off when we start packing then tick all the items off as we go. They include things like sunglasses, spare glasses, spare belt, spare bootlaces, travel kettle, alarm clock, sunlotion, tickets and passports, medicines, headache pills, shaver etc. etc. Once everything is ticked off I know we haven't forgotten anything.