Obviously, long sleeve shirts and long pants, my recommendation is ...
I agree with Bajaracer's basic recommendation for long sleeves shirts and trousers. There are many good brands out there and a variety of styles. I currently wear or carry gear from Mont, Snowgum, Macpac, Ex Officio, Paddy Pallin, Kathmandu, OR, Columbia and North Face to name a few. Generally I keep an eye out for bargains at sales, rather than shop for specific brands.
What I do want is material that dries quickly. Colour selecion should suit the season you are walking in - if you are walking in summer, you might want to choose lighter colours.
For trousers, I prefer styles that have zippered pockets in the legs rather than baggy cargo style pockets all over the place. I find they just get in the way when I am using walking poles. Similarly, I prefer zippered rear pockets, although when I am travelling, I avoid putting anything in these and keep things in front of me. The best trekking trousers I have are from Mont. These have normal slash pockets on both sides, and two pockets on the front of the thigh that zipper closed. These are big enough for a wallet, guidebook or folded map.
For travelling, I prefer a loose shirt with enough room in the pockets for tickets, boarding pass, passport, phone, etc. I have carried neck pouches in the past, but find the slightly baggier pockets on some Ex-Officio travel shirts have enough room, as well as an inner pocket on one side with a zipper for items I want to be a bit more secure. These are a bit of a compromise for trekking, because the pack straps cover the pocket opening. The best shirts I have there have a vertical opening in the centre of the chest that is closed with a zipper. This allows easy access even with a pack on.
I also wear mitts, similar to bicycling mitts but without the padding in the palm, or if it is colder, a full glove. This protects what would be other the otherwise exposed backs of my hands.
In 2010, I wore a wide-brimmed hat, but plan to use one with a medium width brim next year.
Regards