Make sure to put something on the medium/heavy side on top of your perler immediately after fuse; if you don’t, the will warp at the edges, and you will have to heat it back up to get it to lay flat again. I’ve seen other people use heavier items like dictionaries, but I don’t see the benefit. I’ve had success with DVDs and smallish books.
If you can, take your creation off the pegboard before it’s cooled (don’t burn your fingers!) and lay it on a piece of parchment paper, give it a few passes with the iron to heat it up a little before putting another piece of parchment on top, and then applying the weight.
It doesn’t take much weight, all you really need is a couple of DVDs or books. 15-30 minutes should be fine to let it cool.