I lay everything out on the bed and then remove stuff that I dont really need or that I wont realistically wear (do I actually need a pair of jeans or that pair of wedge shoes that only get worn once
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). I then see what im left with. If its still too much then will do another sweep and get rid of more. I always take too much and after a holiday where I only had 15kg luggage allowance it was apparent that I had to be ruthless. I managed with no problem with half my usual stuff so now always pack on the basis of 15kg for whatever duration of holiday (it also gives me space on the way home for any goodies that I want to buy
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).
I do wear dresses more than once whilst im away so in my head they count as double (ie if I take 3 dresses that enough for 6 days!). I mix and match too so (eg) a pair of navy cropped trousers has 2 tops that will go with it (2 days wear). Already, with my examples, I have 8 days' worth of clothes with just 6 items. The only person that does get clean clothes for everyday is my 7 year old, kids can be so messy so I cant rely on her able to wear something twice. One thing im adamant about however, is I always take enough clean underwear for EVERY day, cant be doing with washing knickers on holiday
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but if you buy new, its all packed into a nice little pouch and doesnt take up that much room