General Clothing Tips

1. Try to take only nylon or polyester (including fleece) garments, other than your outer garments, which should be waterproof and breathable. Avoid cotton because it really gets clammy, and wool because it's heavy and gets even heavier when wet. Even undergarments should be poly or nylon. You can wash these out anywhere and most will dry overnight, too.
2. Always have a light-weight waterproof-breathable shell. Gore-Tex is the most popular and probably the best. Get one that's a bit large for you so you can push stuff under it (like camera and binoculars) when it rains. When you expect a decent amount of rain, or if you have to hike in it, consider getting yourself waterproof pants. I always take them along.
3. I don't have any special brand preference on my gear. I have some North Face, some Patagonia, some store-brand from REI and EMS, a few Marmot items, and so forth. Frankly, I buy most of my stuff at online outlets or sales. Almost any decent brand is going to be as good as you need. Just check the labels to be sure that you're getting what you need in terms of the material the garment is made from. You may want to get a larger size than you normally wear so there's room to cover gear you might be carrying.
4. Know the conditions where you're heading (see weather sites above) but also your expected level of physical activity. You don't want to over-dress and sweat while hiking.

Hot-Weather Trips

1. Be sure you take two water bottles unless you're sure you are getting them on the trip. Also be sure you know how you're going to carry them. Consider salt tablets if you sweat a lot; check with your doctor or rely on your own past experience.
2. You will probably find insects in hot weather, so take a good repellent.
3. Consider light-weight nylon convertible pants and absolutely don't take cotton pants, shorts, or jeans! If insects are a problem, keep the legs zipped in.
4. For your shirt, the most important thing is that it's lightweight material and breaths well. Nylon is good. Some people (myself included) like long-sleeved shirts with button-up sleeves, some like shortsleeves and others like the athletic tops, sleeveless. The only issue with sleeves is that if you don't have them you have another area to put bug repellent and maybe sunscreen on. I find that the sleeveless tops don't always do well because you can chafe your underarms with pack straps or life jackets used in landings.
5. For footwear, you may want to find a good water sandal you can wear while hiking and that you can cross streams and swampy ground with. Even waterproof boots will fill with water if you get in deep enough. Wear poly short socks unless you're sure your feet don't blister when you're sock-less. I like Keen H2s; they have a closed toe that will protect you from rocks and also from sharp branches and roots when you wear them hiking. However, don't expect to do long hikes in water sandals unless you've done some hiking in them and broken them (and your feet) in.
6. Take anti-fungal foot powder or a general medicated powder and use it even if you change socks or wash and dry them regularly.
7. You may want to take a general medicated powder for your neck, underarms, etc. Some people will get a local rash pretty easily and it's hard to get rid of it when you're always sweating.
8. You will need a hat to reduce the chances of sunstroke unless you're always walking in the shade. Brimmed hats keep the sun off your face and neck better, and you can also use a net cover if bugs are bad. If you like a cap, consider the ones with a little snap-on cape in the back to protect your neck.

Cold-Weather Trips

1. Add warmth in the form of layers. If its' going to be chilly at night, take a fleece jacket (the windproof kind is nice, but your shell will serve for that if necessary). If it's going to be cold, take heavyweight poly turtlenecks, poly winter undershirt, and a heavier fleece or insulated jacket. I also like to have a heavy fleece vest to wear. Generally, the insulated "jackets" (thermaloft, primaloft) in the lighter weights are easier to pack and warmer than fleece. If you chill easily, get one!
2. Forget fashion, wear a hat. You will lose a ton of body heat through your head. I never wear a hat except on trips, and there I always wear one.
3. Pick gloves carefully for the conditions. I take a lot of pictures, and so I have fleece mittens whose finger flap folds back to reveal half-glove fingers underneath. I have two pairs, one a size too large so I can wear light waterproof gloves under them. The other I can wear with light poly under-gloves and use those chemical handwarmers in each glove for extra warmth. Be very careful about how dexterous you are in the gloves or mitts; you'll need to be able to work your camera, etc. It's nice to have waterproof gloves if you expect rain.
4. Boots are essential for any adventure. Don't even think about sneakers, avoid low-tops if you are in mud, snow, or rocky areas. Good hiking boots that are Gore-Tex lined and thus waterproof will make you a lot happier. If you are going out in very cold areas, watch the temperature rating of the boots. Most people find that something that is "rated" to a given temperature is darn uncomfortable even near that point. I find that -40-degree rated boots work well down to maybe -20 but are cold thereafter.
5. Get chemical warmer packs for gloves and feet if you are going into temperatures that are regularly below freezing. You can buy them in any outdoor store. If you are careful with sizing you can stick them in your boots. Trust me; without something like footwarmers in your boots even -40 boots won't keep you warm if it gets really cold, and your hands will be hopeless even at 10 degrees without some handwarmers.

My Personal Options, Warm and Cold