If you heat the toner it will melt. Take the printed paper and put it on an iron for clothes, see if it melts.
There is another method, use acetone mixed with isopropyl alcohol. It requires no heating. But the acetone must be pure, not mixed with water.
It is very important to heat and press it uniformly(heat), or press it uniformly(acetone method). Use warm water to remove the paper. Let the water completely soak the paper.
This can be very frustrating, indeed. I tried heat with an iron and also a hot laminator to no avail. For what it's worth, here's what worked best for me:
Print the circuit on 70# litho that is coated on one side. A good print shop should know it will work safely in laser printers. They may know it as it 70# C1S Gloss Text. Print on the glossy side. Other papers may work as well, as long as the surface is super smooth.
Create a softening mixture of 2 parts rubbing alcohol to one part undiluted acetone.
Clean the board with #0000 steel wool or emory cloth, and wipe clean with pure acetone or alcohol. If you touched the toner on the paper, use a damp paper towel to wipe it with alcohol.
Place a paper towel down, and lay the copper clad on it, copper side up.
Have the transfer sheet ready, and drip some softener on the board so that it covers 1/2 to 3/4 of the surface, and quickly spread it around with a finger to fully coat.
Place your transfer, toner side down, on top of the "puddle" of softener, making sure it is in full contact, but "floating," and wait about 10 seconds.
Now, press down on one edge a little firmly, and gently smooth the rest of the sheet. Some fluid will run out of the edges, and the sheet will become slightly transparent. If you can see the pattern through the sheet, it is probably wet enough to transfer. If there are non-transparent spots, just add a drop of softener, and spread it around.
Now, lay a folded paper towel over top and press straight down firmly, but not too hard, with the palm of your hand. This is the real trick. Too light, and it will not stick to the copper. To hard, and the toner will spread. It took me about a dozen tries to get the "feel" for it. Be patient, and start with smaller boards to hone the technique.
Remove the board from the towels to dry, and go and get something to drink. When you return, smell the board. If you no longer smell the acetone, it's time for the next step.
Submerge the board into a tray of water and leave it for about an hour. Sometimes it's less, but allowing this much time will ensure the paper is good and soaked, and will eliminate the variable. Check it every ten minutes, and you will get the idea of how it looks when it's actually ready.
After the hour, peel the paper off from one corner. You will get almost all of it off, and then have to remove the remaining pieces.
Hold the board under warm running water, and gently rub the remaining paper away with your finger tips. When you are done, there will be a layer of paper remaining on the toner. This is fine.
Etch using your favorite method, and clean the toner off with straight acetone on a towel.
The trick is getting the right amount of pressure, and being patient. Most of my problems with this method were related to those two issues. That, and understanding that the paper coating will remain on the toner. Knowing this, I have also used regular printer paper successfully, but the fact that it is not the smooth "laser" paper caused it to be a little rough to be usable. Since I got the technique down, 9 out of 10 of my boards are perfect.