Ooh! Currently planning this myself, the wardrobe planning is half the fun of a holiday!!
Most important thing is to choose a few colours (white, grey, navy, red & tan for me) and have everything in those shades so it all goes together and you can mix and match.
Rule 1: Get the swimwear sorted first, it's the hardest part. Bravissimo/Figleaves all the way.
Rule 2: Have plenty of beachwear coverups as they get dirty/salty very quickly. Need at least 3 kaftans/maxidresses/big white overshirt type things. You will find you pretty much live in these. Buy cheap.
Rule 3: You only need one 'nice outfit' for restaurants etc, no-one else cares that you're wearing the same thing as last night. Certainly not husband.
. It should be something that does not need ironing (I'm looking at this)
I have spent £50 on a new bikini to add to the 3 other swimsuits I will take. I will spend £150 on the best sandals that I can afford. These are investments and will last for years.
I will spend maybe £30 at Primark/Wallis/Debenhams/Dorothy Perkins for all the rest - new kaftan/cover-up stuff and a couple of white vests. Everything else I have already.
...And will spend £20 on a bottle of Clarins sunscreen at the airport duty free as it makes me feel like a rich jetsetting Eurotrash hotty in a way that Boots sunscreen simply does not. Have a great holiday!