Clothes

Where you will be backpacking and the time of year will dictate what you need for clothes, though any 3 season backpacker from any area in North America other than someone from the extreme northern climes will have approximately the same list.  You won't need to take everything on every hike, and those hiking in the Appalachians will generally be able to pack lighter in the summer than hikers in the Sierras, Rockies, or Cascades. Most experts say the key is to bring layers and add or remove them as needed depending upon weather.

Pants - Convertible hiking pants are the best way to save space and weight as they convert from long pants to shorts in seconds via zippers just above the knee.  They are lightweight, some are water resistant, and they generally have lots of cargo pockets.  I recommend those with zippers at the bottom so they can be opened for cool down when it is too hot for long pants and too cold for shorts.  REI, North Face, Columbia and others make them, and they generally retail for around $55, though they can sometimes be found on sale for $39 or less.

Shorts - Though convertibles may be the best way to go, every backpacker should have a pair of shorts for warm trips and swimming.  Those same manufacturers same shorts in the same material, and again, cargo versions can be helpful.  You may decide to carry a swimsuit as well, but that's up to you.

Shirts - You'll need both long and short sleeve, and something very light and breathable for warm days and something a bit thicker for cooler days.  All shirts should be of the type that wick moisture away from the body.  Tech Tees are good all purpose shirts that come in several fabrics and both long and short sleeve.  Running shirts are also popular, and lightweight button up shirts such as fishing shirts with vented mesh backs are also a good option.  You can use your favorite standard T-Shirts such from your favorite sports team, pub or whatever, but they typically don't wick sweat or dry out as fast as the other options.  I like Columbia's Omni-Dry shirts.

2nd layer - A lightweight fleece that is worn somewhat loosely or tighter fitting "skin" made of polyester knits each both provide about the same amount of warmth for the trail, and are perfect for nightime around the campfire when temperatures start to cool but a heavier jacket is too much.  You could even create 3 layers via a t-shirt, the skin, and the fleece on top, and still be less encumbered than with a heavy jacket.

Jacket - If you're above 5000' or are outside of the warmest summer months, a jacket is a requirement.  A number of companies make light down jackets, and as pointed out in an earlier post, I went with the Marmot Zeus.  Just remember to keep far enough back from the fire that sparks won't land on it, because the fabric will melt.  Most of these jackets either stuff into a pocket or come with a small stuff sack, so the jacket can double as your pillow.  Some packers also (or in lieu of) carry a down vest.

Rain Protection - You have to have it as even in California, Sierra summer storms can hit at any time.  There are several choices from a cheap thin baggie-type poncho with hood (I have my Niagara Falls Maid-of-the-Mist poncho in my day pack), to a standard lightweight rain jacket to ponchos specifically designed for backpacking which normally cover the pack as well as yourself.  I purchased the Outdoor Products PackFrame Poncho (normally $33, but I paid about $25), which does cover the pack and can double as a tarp or lean-to shelter.  There are far lighter and more expensive backpacking ponchos/tarps on the market as well, so do your research.  Whatever you choose, make sure it has a hood, and if it is just a standard jacket, you will need a rainproof pack cover.

Socks - If you are wearing mid or full height hiking boots, wool socks made by SmartWool are the dominant brand, though there are a few other choices.  They come in a variety of styles, patterns, thicknesses and lengths, and a word of warning - they aren't cheap.  Make sure you pick one that works well with your hiking boots, and it is recommended to have some at the ready when trying on boots.