The people who live in the jungle don't wear much just something around their privates and the women are usually topless! If one is preaching Islam to them how do we tell them to cover up more? (You can reword my question - I'm sure it can be improved)
Assuming that the dawa crew has the Islamic clorhing to supply the tribe since you cannot ask them to wear clothing that is not available :
The first step would be to isolate the men from the women in public areas until they accept Islamic dress.
The sisters will teach the women by example, and the brothers the men.
Changing lifestyle, especially clothing is difficult. It can only be done if the faith is strong. Fortunately all Islamic rules are practical for the dunya as well. The hijab protects against skin cancer, and white hijab can help keep cool. This is in addition to assisting the men in lowering their gaze and fostering a society where it is easy to perform worship and taqwa.