To get used to traveling lightly, travel expert Rick Steves suggests a sort of dress rehearsal -- pack up what you plan to take on vacation, and haul it around for an hour. If the very idea exhausts you, you probably have too much in your bag.
You can get a lot of mileage out of this exercise: It can show you how too much luggage will weigh you down, keeping you from window shopping or walking from a train station to your hotel. On the other hand, if you've truly packed light, you may realize how little you really need to carry with you on vacation.
So how do you cut down your packing list? First, think about every item you intend to pack: Will you use it enough to make it worth hauling around? Is there a smaller, travel-sized version available, or can you just buy it when you get there? Find out if you'll have access to laundry, and if you do, think about leaving behind some T-shirts and underwear. Remember that you're traveling, not socializing with friends, so you can wear the same pair of pants more than once without breaking any social taboos. Stick to slacks or khakis if you're traveling in the summer -- if it's hot, you'll melt in jeans.