If there's one thing that you should keep in mind when packing for your trip to Phuket, that would be not to pack your entire wardrobe. Phuket has a pretty easy-going vibe, where you could throw on a pair of shorts, a tank top, and flip-flops and walk around all day long without getting any weird looks.
Given the blistering hot climate of the island, lightweight, warm-weather clothes should take up a large chunk of space in your backpack. A mix of 3-4 t-shirts and tank tops, 2-3 pairs of trousers and shorts, a skirt and a dress (for the ladies, of course), would be an ideal travel wardrobe for Phuket.