Spending 6 months traveling the world, you learn what you need (in your closet and on the road). Many a vacation has been ruined by poor packing. Whether you forgot to check the weather or threw out your back lugging around a giant suitcase, poor packing can lead to a stressful vacation.
It may sound silly to remind you to check the weather but, when you are rushing to pack the night before a vacation, it may slip your mind. Also, some people just assume the desert will be hot and forget to pack a sweater for the chilly nights. There will always be unusual weather that you can't plan for but it's important to have a general idea of what is needed for each destination.
The easiest way to keep your suitcase light is to color coordinate. Try to make sure everything you pack goes together. It doesn't all have to be the same color but choose colors that go together. That way you don't need to pack multiple belts, shoes, purses, makeup, etc.
As mentioned earlier, people often forget to bring a sweater for cold desert nights. Does it kill to you pack a bulky sweater when the rest of your bag is full of t-shirts? Layering is a great space saver. Pack a few light long sleeves and multiple light layers will usually do the job.
Planning to visit churches in Rome or temples in Thailand during the hot summer months? While you may plan to wear sleeveless shirts and shorts to beat the heat, you may be turned away from visiting these sites. Know the dress codes of the key sites you plan to visit as religion sites often require covered knees and shoulders. The good news is that, even if you forget to pack your own, there are usually venders selling shawls or even pants rentals
Are you a big shopper? Planning to get custom made clothes made in India? Do you bring presents back for everyone? Invest in a cheap duffle. It doesn't take up space on the way there and you can unfold it to stuff it full of all of your goodies on your return.
Does it look like you're going away for a month when, in fact, you're escaping for a long weekend? Take a minutes to look at your bag before you go and think about what you really need. It's nice to pack an extra top or pair of underwear just in case but don't go overboard. You may be happy you are prepared for every situation but you'll be miserable lugging everything around. Put everything you think you want to bring together and then edit. If you're going on a 2 week tour, you don't need to pack 2 weeks of clothes. Pack a week's worth of clothes and do laundry half way through your trip. Many hotels will do laundry for you or can help you find somewhere at your destination.
It's easier to pack less if your bag is organized. I'm a big fan of packing cubes. putting all of your pants in one cube and underwear in another allows you to see what you have and, even better, allows you to find things quickly while traveling without emptying your entire bag.
Don't stress too much, though. You can always go buy something you forgot to pack. Shopping is a universally loved pastime.