As everyone should know: avoid cotton like the plague. Polyester rules.
I am a huge fan of Patagonia gear, so many of my recommendations will be their stuff, but you should be able to find equivalents in other brands. Silkweight capilene is a good solid baselayer recommendation for warm weather, heavier weights for colder weather. I often pack a thermal pants layer that I use if nothing else for "pajamas", but it makes good insurance in case the weather does something surprising (Cap 2 bottoms weigh 5.5 ounces). Thermal pants with a pair of "baggies" shorts worn over them is a workable setup that avoids sunburn.
I also like wearing thermals under something like Baggies shorts when starting out on a cool morning, then I can peel off the thermals later when the day warms up. Watch for sunburn. Baggies shorts are 8 ounces. I like the "Ultra" shorts (7 ounces), which are polyester instead of nylon, but the nylon Baggies seem more indestructable.
Upper layers: I strive for thin light colored layers with long sleeves for sun protection. My favorite has become the Mountain Hardware "Canyon Shirt". I own several (but select just one for a given trip).
My first reaction was the price tag -- Tilley Hats are expensive. But now I am a total convert, and realize that every other hat I bought was wasted money. The instructions for the Tilley Hat tell you to wash it, and to wash it often. Just reading that made me a convert. I owned a LTM-6 hat (an "airflow" nylon model) for about 3 years, then lost it in a talus field. I immediately bought another LTM-6 to replace it. I first bought size 7-5/8, but quickly went back and exchanged it for a 7-3/4. Their advice on the fit is that you should be able to insert 2 fingers between the hat and your forehead and that the hat should rotate without friction. This is good advice, along with my comment that the hat should just set onto your head with the least bit of friction. As they say, use the strap - not friction, to keep the hat on in the wind.
The original hat had been washed many times, was the most comfortable and excellent hat I ever owned. You get what you pay for.
Lately I have grown to like the Outdoor Research "sun runner" hat, which is like a light baseball cap with a flap thing that hangs down over your ears and the back of your neck. This is better than anything else I know for sun protection, and I can almost dispense with sunscreen when I wear it.
I purchased a pair of North Face pants with zip off legs and they fit me wonderfully. I got a terrible sunburn on my calves the first time I tried the zip off thing, so by and large they are just a nice pair of pants with useless zippers providing extra weight, but a good option nonetheless.
Lately I have been wearing Prana's "stretch Zion" pants, which are serving very well.
Tom's hiking pages / tom@mmto.org