Determine how many days you are going to be away from home. At a minimum, you should plan on having two different "looks" per day. Typically, this translates to an outfit for daytime activities and an outfit for evening. Remember, different "look" doesn't mean you have to bring a different combination of clothes for each day. You can mix and match a few items to produce various looks. For example, you can wear a single outfit day and night while changing the "look" by adding scarves, accessories or a jacket.
Identify what kind of activities you'll be engaged in during the trip. If it's a backpacking trek to Patagonia, for example, you're probably not going to need evening gowns and dancing shoes. If you plan to be in business meetings all day, your wardrobe will revolve around fresh shirts and several different ties that can be worn with the same suit or tailored slacks and a sports coat.
Determine how much luggage you're going to need. Depending on your chosen mode of transportation, you may be limited in terms of the number of pieces of luggage you can check through. In addition, oversize bags may be subject to additional fees due to weight restrictions.
Build your travel wardrobe around one basic color that can be mixed and matched with complementary and/or contrasting colors. For instance, if you pack a pair of black slacks, a black skirt, a black sweater, and a little black dress as your core pieces, you can create several dozen different looks by pairing them with solid or patterned shirts in bolds or pastels, jeans, jackets, and scarves.
Choose fabrics such as microfiber travel knits, spandex and cotton gauze that will resist wrinkles even if you spend hours on an airplane. No one really wants to spend half her vacation ironing and re-ironing the linen skirt or jacket that gets creases in it five minutes after she puts it on. Leave it at home in the closet. Choose something that won't make you look as if you slept in your clothes.
Choose fabrics that are stain-resistant. If you're unsure, opt for items that are dark and patterned as opposed to clothing that is light and solid. If your trip is going to be a lengthy one, choose items that can be worn multiple times. Pick items that can be rinsed out in your hotel room.
Start planning your travel list with grid paper and a pen. Down the left side of the page, list the bottoms you want to take. Across the top of the page, list all of the individual tops. For each item you have listed down the left side of the page, you should be able to match it with at least three items from the top bar. If, for example, the jersey knit skirt you have listed can only be stylishly paired with one specific top, you'll need to rethink one or the other.
Choose two pairs of shoes. Pick one pair that is dressy and one that is casual. Both pairs should match most of the things that are in your wardrobe. Shoes tend to take up a lot of room in suitcases, and they are also heavy. Keeping pairs of shoes to a minimum is wise.
Hang up or set aside the clothes you plan to take on the trip. Do this a couple of days before your trip. This will save you from scrambling at the last minute.
Remember to pack your underwear, socks, nightwear and toiletries.