Rubber boots are a must and are almost always provided at the lodge or through your tour company, up to size 10 (US), or can be purchased in advance. Also bring: insect repellent (with DEET); malaria pills; antihistamine tablets and an epi-pen for people with serious allergies to stings; binoculars (invaluable in the rainforest - it's worth spending a bit of extra money to get a good pair -8 x 40 are excellent for poor light conditions under the forest canopy); plastic bags for keeping your clothes dry; swimming suit; lightweight, quick-drying clothes; at least one long-sleeved shirt; one pair of loose-fitting pants (no jeans); a light sweater (it gets surprisingly chilly in the rainforest, especially on boat trips); poncho that fits over you and your pack (the cheap plastic knee-length type coats are better than Gore-tex, which will soak right through in a real rainforest deluge); bandanna; at least one pair of socks per day - or more; Teva-like sandals or sneakers for around camp; and Ziploc bags for food, books, maps and anything else you hope to keep dry.
All clothes (undergarments included) should be loose fitting to help keep you cool and to reduce your chances of being bitten by chiggers.