I used to have the same problem, the edges. I learned the center of the iron seemed to be a little hotter the outter edges. All the info here right on, but that was my edge problem..
1) make sure it copper is clean, I use denatured alcohol after I wet sand the board. (acetone to remove the toner later works great)
2) no burrs on the edges
3) iron on max heat
4) iron for a couple minutes (first start by just sitting the iron on the board), and use the center of the iron to go around the edges and go slow.
I also cut the sheet to the size of the copper.
I get perfect boards everytime (until I drill them
) and I use photo paper, the pnp is a lot less work, but expensive when you have to throw 80% of the sheet away.
But this works for both