During my stay in Peru, I didn't travel to the rainforest, but I would think it is best to dress in layers depending on the weather and time of year travelled there (nothing too heavy as it will probably get warm there). Also, definitely important to have bug spray/mosquito repellent or some kind of netting for protection from the different insects in the rainforest.
I did my volunteer abroad 7 years ago and at the Animal Conservation in Peru so my perspective may be a little different, but while I was there volunteers are supervised to an extent. However volunteers are left alone when not working which is quite often, and its hard for new volunteers who don't know much of the area. Doing more group activities not related with volunteering would help people...
During my stay in Peru, I didn't travel to the rainforest, but I would think it is best to dress in layers depending on the weather and time of year travelled there (nothing too heavy as it will probably get warm there). Also, definitely important to have bug spray/mosquito repellent or some kind of netting for protection from the different insects in the rainforest.