When packing for such a long trip, it's important to remember: they will have laundromats and dry cleaners where you're going (unless you are going to the Outback of Australia or the outskirts of Kenya.) The important points to keep in mind are to mix and match, keep it simple and don't bring anything that can be bought inexpensively abroad or wherever you'll land in a few days.
Don't overpack. While opinions on this differ, for those of us of the pack-light mentality rewards are plenty: less hassle at airports, fewer backaches from lugging around excess baggage, quicker boarding times, even less cost getting on board due to the weight of our suitcases (sometimes). So start by thinking light. It will transcend the physical into your overall outlook. Thinking light means having fun, allowing experience to engulf you--not to be bogged down by every material possession you own.
Get two good bags - one big with rollers, one a small duffel bag or backpack. You will appreciate the wheels when you're waiting for hours on end at the airport.. However, it's also good to have a carry-on. First, there may be items you need while on board (or on the train or even bus); second, airlines lose luggage all the time. You must always keep your valuables with you. Don't trust the airlines to get it right. A good percentage of the time things go wrong, so much so that most regular travelers have at least one horror story about lost luggage.
Think mix-and-match. For ladies this means a few basic skirts and pants or shorts such as khakis, jeans and basic black wools or cottons (depending on the time of year and where you are going.) Throw in a few fun bits (your favorite Hawaiian sundress when you're going to a tropical locale or simply want to think tropics when you're in Milwaukee all summer) but don't go overboard. You will definitely appreciate the one fun dress you brought along much more than you'll value the excess you packed. There's nothing worse than realizing you never wore half the items you brought, have now bought more to add to the wardrobe and are literally sitting on your suitcase to get it to close.
Remember everyday items other than clothes. With the exception of items that cost under $10 (such as corkscrews and nail clippers) you don't want to be stuck for five months without your doctor's prescription for your nervous stomach. Your insurance may not allow you to go to a doctor at your destination (without penalty) and it could be problematic to get another prescription. Make sure you bring enough pills along to last the five months. Bring your favorite book, journal, and a couple of DVDs. Don't forget your laptop, especially if you are a writer or are the chatty e-mail type. If you won't miss it, don't bring it, of course. Items such as a favorite pillow or stuffed animal are not to be dismissed. When far from home, a familiar scent on a pillow and the way it folds between your arms can ease anxiety during travel.
Make a checklist of what you need and double check that all was packed. You can lay out all the clothes and other items you want to bring then go over a list or vice-versa. The important thing is to account for everything in your suitcase(s) or you will definitely wind up missing something or have three more pairs of shorts than you need for five months in Bermuda.