The number one rule to abide by is to know the place you are going. Not the destination, the climate. If you are going to Alaska, pack warm. If you are going to be hopping from island to island in Hawaii, your wardrobe should be a tad lighter.
A trip is an occasion for many things. Buying a new wardrobe is not one of them. You are going to find yourself walking a lot more than usual. As such, comfort is the name of the game. Pack things that are broken in and you are used to or you will regret it.
While you are probably not going to run into friends or family abroad, you don't want to be a fashion nightmare. To avoid such problem, try to pack one color of clothing along with some neutral colors like black. It might not look great, but it won't look terrible.
You have undoubtedly thought about how you will carry valuables such as cash and your passport. There are a variety of approaches, but don't wear a fanny pack. Fanny packs scream "steal me" to thieves.
If you are going to be in a cooler climate, think layers when it comes to your clothes. If you're going to be expending energy, you are going to start out cold and then get hot. If you dress with layers, you can peal them off as you heat up.
Safari vest are used frequently by travelers and for sale just about everywhere. Okay, they are pretty ugly, but they are definitely a good practical item. Make sure you get zippers on the pockets so nothing falls out and nobody picks any valuables from them.
What about keeping your hands warm in cold climates? If it is going to be seriously cold, mittens are better than gloves. Why? You can hold your fingers together, and they help keep each other warm. Learned that one in Russia.
Society is very much into bling-bling these days. So, what should you take on your trip? Unless you are going on a luxury excursion, I suggest you take nothing. Why become a target for thieves? If you must, take fakes. It is not like you are going to see friends.
If you get frustrated when you start packing, just take a break and go watch television or something. As shocking as this may sound, much of the world has the same things for sale, so you can find anything you forget at your destination.