To use a rather cliched phrase, "capsule wardrobe" Stick to possibly 3 colours, make sure everything will go with at least 2 other things, then bung in some "accent" stuff, i.e. scarves, necklaces, earrings etc.
Start with the shoes, ones that you have worn before and know to be comfortable!
(Get me, the stylist!
)
I actually did this on a 5 day trip to Paris, and it worked! But I had sort of cheated, found some clothes I liked, bought the top, which was quite a long shirt style, with 3/4 length sleeves, the skirt (long) and the trousers, all in a dark blue print, packed a couple of white tops, jeans and my beloved classic blazer, everything went with everything else, was able to sort of mix and match stuff, and bingo! Bob's your uncle's milkman's budgie!