Don't Pack at the Last Minute
Last-minute panic packing leads to a messy suitcase, forgotten essentials, and mismatched clothing. Start packing early so you're not in a bind if a piece of clothing you need is dirty or you need to buy something for the trip. This also gives you time to trim down after you've started packing.
Plan all of your outfits ahead of time. Decide on one outfit per day (or per occasion, if you will need multiple outfits for each day). Don't pack any more than what you need for each outfit.