A common rule for "How much stuff do I need to take?" is to set aside what you need, take half of that, and then plan on buying clothes that you need if and when you need them. I have found that this doesn't always work for me. I think you have to consider the country you are going to. For example, many countries have high quality and inexpensive clothes that you could easily supplement your wardrobe with. As well, think about when you are going to be traveling. Cooler climates will demand more (and bulkier) clothing while traveling in warm regions allows you take only the basics like shorts, dresses and t-shirts. Because clothing will take up the most room in your pack, take less and plan on washing what you have more often.
Choose clothing that will allow a minimum amount of items to be carried. This is done by selecting items that can be mixed and matched together, based on colors or styles, to create a variety of outfits. Choose the basic essentials that you will need but ensure that they are versatile enough to be easily combined. Choose muted colors that are easier to care for and allow you to blend into the crowds a little more.
Try to choose functional clothing items that will offer protection from the elements yet will be comfortable and easy to care for. You need low-maintenance clothing that will be flexible enough to keep you warm and dry, if need be, yet also be compact and light enough to carry in your pack. Natural fibers like cotton don't wick sweat away from your body very well and dry very slowly, but are warm and easy to care for. Synthetics like polyester are great at wicking moisture away from your body and dry quickly, but can be expensive. Aim for a combination of these to accommodate all your travel needs.
Shorts and t-shirts are essentials on almost any trip. However, on some occasions, you will need to wear pants or cover your arms and shoulders, such as in some churches or holy places. Women in particular should be prepared to 'cover-up' in some countries (such as Muslim countries). Wearing revealing clothing could be unsafe to the wearer or be disrespectful to the locals. Also, in some larger cities, shorts are not worn at all by the locals (even in warmer climates). By wearing shorts yourself, you would stick out as a foreigner right away.
The key to staying warm when the temperature drops is to layer your clothing. Rather than pack bulky or heavy items, simply take a few lighter and thinner pieces that can be worn together. For example, a t-shirt combined with a fleece vest and a wind breaker or rain shell will keep you relatively warm and dry. These are all light pieces that can be worn on their own or all together to offer you the protection you need, depending on the weather.
One final note that needs to be mentioned - you can buy almost any piece of clothing anywhere in the world. Don't take something "just in case you might need it". If you happen to go to a cooler country, chances are they will sell warmer clothing. The same is true for countries with warmer climates. In both cases, take clothing that will prepare you for the general climate, but be realistic.
Rather than take many pairs of socks,underwear and t-shirts, plan on doing some laundry while you are away. This is not very difficult to do, and will allow you to pack lighter. If you aren't into doing laundry too often, take some older clothing that you are almost ready to throw away. Then, when it gets too dirty to wear, discard it and buy some new items as needed.
Be prepared to do your laundry by hand. It can be difficult to find a convenient laundromat. And even if you do find one, it will probably be expensive and will definitely be time consuming. Instead, just wash your clothes in the sink of your hostel (or even at a lake or river if need be). Purchase a sink stopper (use a racquetball) and take some detergent in a Ziploc bag and you are all set. If washing in a natural water source, use biodegradable liquid soap that won't hurt the environment (please!). Some soaps can be used to wash both cooking gear and clothing. In either case, you can also use shampoo or bar soap to do laundry.
Your undergarments will probably be the items you wash most often. Stick your hands in your socks and rub them together. Shirts and pants can be cleaned by rubbing different sections together. Hang your clothing to dry them out or, if you are in hot climates, just put them on to keep you cool and your clothes will dry even quicker. Remember that cotton takes longer to dry than synthetics.