The trick to toner transfer is to get all the toner hot enough to become
"sticky" but not hot enough to actually melt. There's a small range of
temperatures for each specific toner; the closer you can get to (and
hold) this temperature, the better.
I did a few experiments with transparencies...
http://www.delorie.com/pcb/transparencies/The copper needs to be clean enough to let the toner stick ∗and∗ needs
to be "roughed up" with an appropriate grit sandpaper so the toner can
get a mechanical bond with the copper.