So I was working on my second hat, which is similar to the first one except, this time I wasn't so lazy about purling and decided to do a proper knit/purl double knit (mostly cause the hat is for someone else).
Unfortunately I very quickly noticed that my purling is a -lot- looser than my knitting, which means that the "inside hat" is bigger than the outside and bunches up. This may not be such a big issue in a hat, but it pretty much rules out ever trying to knit two socks at the same time. I'm not even sure whether this is because I hold my purling yarn on my middle finger (as opposed to my index) or whether my purling is just naturally looser or if it's a combination of the two.
So I went to my good friend Google to try and find a solution and found some mention that the Combination Purl Stitch is tighter. In a nutshell, this involves wrapping the yarn clockwise around the needle instead of counter-clockwise (ie. from the bottom up). On the next row you then have to purl into the back of those stitches (it's -much- easier than purling into the back of a normal stitch, promise).
The photo below is of the inside of the hat (ie. the purl side) and you can (hopefully) see the stitches tightening up around the middle of the mushroom's "hat".
Hope this can help someone.