Hi,
I've been trying out a technique I found in an art book. This was relatively easy and I think it looks good. Also involves no painting.
I used plasticine to make the mushroom and then coated it with PVA glue. The glue makes it glossy and also harder. I have only applied 2 layers of glue to this one so far. i think the more you put on the harder it would get (which would help it from being squished.)