It really depends on how "extreme" where you are going is but I would agree with wild, you need strong boots, preferably goretex, long trousers that dry quickly (eg rohans) and t shirts that aren't white as anything white looks manky very quickly. You will need a good waterproof and possibly a waterproof cover for your rucksack if you are trekking and camping. Don't forget good insect repellant.
I've got craghoppers stuff that I've used in Australian rainforest. It's very lightweight, dries quickly and packs into very little space. I have got long sleeved shirts and trousers (which I tuck into my socks - sod style, keep the biting buggers out!). I get the 'nosilife' stuff which is impregnated with mosquito repellent, but you will need insect spray as well. Currently lots (up to) 70% off at craghoppers - www.craghoppers.com/ and code 'CE64' may give an extra 20% off and free postage (it was working at the end of last week).