Okay, let's face it, if you spend anytime hiking, you are going to get wet. Whether it is a warm day and you are perspiring (I sweat, but some of you might prefer that term) or it's raining or both, being wet is a given. Now that we have accepted that reality, let's talk about how to deal with it.
First and foremost, remember this: "Cotton kills!" It is also uncomfortable when it gets wet. When I hike, I generally have only two pieces of cotton in my attire: a handkerchief as a sweatband and maybe underwear, if I am just out for the day. Cotton, which is comfortable when dry, causes problems out in the mountains because it holds moisture against your skin. There is some real science here (a great article by one of my favorite hiking bloggers on the subject), but the thing to remember is that wet cotton 1) doesn't dry fast enough and 2) doesn't insulate; in fact, because it holds the moisture and doesn't insulate, it has a negative insulating factor. In the case of the handkerchief, I want the negative insulating factor; it is one way I deal with overheating, but it is the only place I will wear cotton (save the undies, sometimes).
So, obviously, you will need a shirt that isn't cotton. Your core is the most critical important part of your body for well being, and the shirt you put against it is an important choice. In the summer months, you want a shirt that will pull the moisture away from your skin, will provide insulation when wet and will dry quickly. There are many shirts out there that will do this. My favorite, though is the UnderArmour Loose T-Shirt. I haven't tried the EMS Techwick or the offerings from any of the other outdoor retalers, and they may be just as good, but I haven't tried them, so I can't recommend them.
Last year, I had family up from California. While they hike, they had never worn technical fabric. I took them to Cabellas before we headed up to Katahdin. The day we hiked was very hot with extreme humidity. As we approached Katahdin Stream Campground on the way down, both my sister and her husband commented on how amazing those Under Armour shirts were- they had never been so comfortable hiking. It really does make a difference.
You also want your pants to dry quickly. In the summer, I wear shots (although I am seriously considering buying a pair RailRiders Eco Mesh Pants). I currently rotate a pair of Columbia Cargo Shorts and a pair of North Face Shorts. Both are made from synthetic fabrics that dry extremely fast. This is very important because when it rains, the bottom of my shorts legs get wet (I don't wear rain pants and all the water from my rain coat ends up on my pants). It also allows me to go swimming without concern if the opportunity presents itself.
Socks are another important area- mostly for comfort in the summer, more for well being in the shoulder seasons and the winter. I wear Smartwool or some other Merino wool blend in the summer. Though my hiking shoes are waterproof, I still want to prepare for wet feet. (Truth be told, I would prefer that my hikers were not waterproof- poor decision on my part.) I thoroughly enjoy walking through that nice cold stream that generally runs beside the trail at some point on the way down, (with my shoes on) but want my feet to be comfortable when I get out. Wool is the natural technical fabric- it insulates just as well when wet as it does when dry.
So there you have it. In addition to maintaining warmth, technical fabrics are significantly more comfortable when you are wet. If you are at all like me, being comfortable makes hiking much more enjoyable.